Chapter XII: Introduction (8)

With respect to the effects of temperature on the range of Cirripedes, no genus (having more than one species) is confined to the torrid zones. Pyrgoma, from being always attached to corals, is, of course, ordinarily found in the hotter seas; but one species ranges from the Cape de Verde Islands in the torrid zone to the southern shores of England and Ireland. Tetraclita is not found in the colder seas, but is numerous in species and in individuals, on the southern shores of Australia and at the Cape of Good Hope. I may here add, that the two genera with the most confined ranges, are Chamæsipho and Elminius; the former has only two species, one inhabiting Australia, and the other the East Indian Archipelago; Elminius has four species, confined to the southern hemisphere, and inhabiting Australia, New Zealand, and South America. To return to the effects of temperature; in Mr. Dana's great work on Crustacea, an excellent chart is given, in which the _isocrymal_ lines, or those exhibiting the mean temperature of the waters along their course, for the coldest thirty consecutive days in any season of the year, are given; and which lines Mr. Dana has shown are the most influential in governing the distribution of marine animals. At the isocryme of 68°, Mr. Dana divides the torrid and sub-torrid zones from the several temperate zones; and at 44°, the temperate from the sub-frigid and frigid zones; but as no Cirripedes are exclusively confined to these frigid zones, we may here disregard them. From Mr. Dana's[76] table of the areas of the torrid and temperate ocean-zones, on both sides of the equator, it seems that they are nearly as 337 to 278, in relative area; and consequently, he remarks, that the marine species in any class, if distributed equally over the two, would be one fifth more numerous in the torrid than in the temperate zones. Now of Cirripedes, taking all the orders, there are at present known 147 species; of these, 7 have doubtful habitats, leaving 140 for comparison. Of these 140, nearly one quarter, or 37, inhabit both the torrid and temperate zones, as above defined; 46 are found exclusively in the torrid, and 57 exclusively in the temperate zones; so that the temperate zones, though less in area, and having, proportionally, even a considerably lesser length of coastline, nevertheless have a preponderance in the number of species. But it should be borne in mind, that there are _two_ great temperate districts, separated from each other by _one_ great torrid district; and, inasmuch as the number of species in any region seems to depend in some degree on the isolation of the sub-regions, we might have expected (the other conditions now being, and the _past_ conditions having been alike), that the two great temperate areas would have contained more species, perhaps doubly more, than the single great torrid area.

[76] 'Crustacea: United States Exploring Expedition,' p. 1476
(corrected).

The proportional numbers, above given, are not very widely different, whether we take separately the Balanidæ, the Lepadidæ, or all together. Mr. Dana has shown[77] at length, that generally amongst the Crustacea, the species which he considers of highest rank, belong to the extra-torrid zones: there seems to me in all such cases to be some degree of vagueness in the attempt to determine which are highest or lowest, but I have already elsewhere stated that Balanus is, probably, the most eminently Cirripedial form, and exhibits in the strongest manner all the characters by which Cirripedes differ from other Crustacea; as this genus is the largest, containing 36 species, of which the habitats are known, I may state that of these, exactly one third, or 12, inhabit both zones; 9 exclusively inhabiting the torrid, and 15 exclusively the temperate zones. According to the proportions of the whole class, the numbers should have been 9 torrid, to only 11.11 temperate; so that evidently the genus Balanus (in one sense typical) inclines towards the temperate regions more strongly than does either the family or the sub-class to which it belongs.

[77] Ibid., p. 1528.

With respect to the relation between the size acquired by the different species of sessile cirripedes, and the temperature of the localities inhabited by them, the genera Chthamalus, Tetraclita, and Balanus, alone can serve for comparison: in Chthamalus much the largest species is found in the temperate zone: on the other hand, the two largest species of Tetraclita are from the torrid zone, though one of them also sometimes ranges into the temperate seas: in Balanus, the largest species, and six other species having a basal diameter sometimes over two inches, inhabit the temperate regions; and two out of these seven species, flourish even in the Arctic seas; whereas, within the torrid zone, there are only three species with a diameter sometimes exceeding two inches, but two of these frequently become very large and massive; so that Balanus, judging from the size of the species, as well as from their range, does not require for its highest development the temperature of the torrid zones.

The greater number of the species of the Balanidæ have wide ranges, as might be inferred from the fact of between one third and one fourth of the total number inhabiting both the torrid and temperate zones; but it should not be overlooked, that those species, as _Balanus tintinnabulum_, _amphitrite_, _improvisus_, and, in a lesser degree, _B. trigonus_ and _Tetraclita radiata_, which seem to range over nearly the whole world (excepting the colder seas), are species which are habitually attached to ships, and which could hardly fail to be widely transported. Indeed, it appears to me surprising, that such species as _Balanus psittacus_ and _eburneus_, which often become attached to vessels, should still be confined, the one to Southern, and the other to Northern America. But some other Cirripedes, which I have never seen attached to vessels, have likewise immense ranges: thus _Tetraclita porosa_ is found in every tropical and warmer sea, and _Chthamalus stellatus_ ranges round the world in the northern hemisphere, and, along the eastern side of America, far south of the equator: _Balanus spongicola_, and _Acasta spongites_, extend from the shores of Britain to the Cape of Good Hope: _Balanus lævis_ ranges from Tierra del Fuego to California. I may further remark, that the only two other species of Balanus, and the one Chthamalus, inhabiting Tierra del Fuego, are, also, found on the shores of Peru. But to show the powers of endurance in some species, I may specify the case of _Balanus improvisus_, which flourishes on the coast of Nova Scotia, amongst the West Indian Islands, in Southern Patagonia, and near Guayaquil. Even more striking is the case of _B. crenatus_, of which I have seen specimens from latitude 74° 48′ north, from the West Indies, and the Cape of Good Hope! In these two latter localities, however, it seems to be rare, and may have been first transported to them from the shores of Europe, on the bottoms of vessels, to which it sometimes adheres.

The several species of Balanidæ live attached either to coast-rocks, or to objects at various depths, down to, as in the case of _Balanus crenatus_, 50 fathoms. _Balanus balanoides_ sometimes adheres to rocks or wood so high up, that it is not covered by water during the neap tides. Mr. Thompson has informed me, that he once accidentally kept some specimens of this species out of water for seven days in a warm room, and that they were then quite lively. This species, is very easily killed by brackish water, as are some other species, whilst _B. improvisus_ and _eburneus_ can flourish in it; and at the Falkland Islands, I saw _Elminius Kingii_ attached to rocks at the mouth of a fresh-water brook, so as to be covered by pure water during the ebb of each tide. Sessile cirripedes adhere to all sorts of objects, floating and fixed, animal and vegetable, living and dead, organic and inorganic. Chthamalus is, perhaps, more commonly attached to rocks than are the other genera. Living Mollusca are, I think, the most frequent objects of attachment: Mr. Cuming has remarked to me, that shells covered by an epidermis, as Patella, Haliotis, and Mytilus, are the greatest favorites. Acasta is always imbedded in sponges, or in the sponge-like bark of Isis; Pyrgoma and Creusia in corals; Chelonobia is attached to turtles, and one species to crabs or very smooth shells; Coronula, Tubicinella, and Xenobalanus, are imbedded in the skin of Cetaceans; and Platylepas in that of manatee, turtles, or sea-snakes.

If we attempt, with our present not very imperfect materials, to divide the globe into provinces, according to the amount of difference in their Cirripedial inhabitants, including all orders and families, and disregarding entirely, as I think we ought, all probabilities or conclusions deduced from the distribution of other tribes of animals, we find that the globe may be divided into the four following great provinces and one sub-province. I should premise, that in the following remarks and tables,[78] the species of Lepas, Conchoderma, Chelonobia, Coronula, Platylepas, and Tubicinella, are excluded, owing to their being attached to floating or swimming objects, and being consequently widely and irregularly distributed.

[78] As the number of Cirripedes in the whole class is not very
great, I have given lists of the species in the four main provinces
and in the one sub-province.

The first, or North Atlantic province, is that of Europe and the eastern shores of North America, from the arctic regions to lat. 30°: the island of Madeira, part of the north-west coast of Africa, and the whole Mediterranean being included. In this province (the above-named genera being excluded) we have 31 species, of which 22 are not found in any other distant quarter of the globe. As some few of these species range into the West Indies, I have not, on this account, excluded them from the 22 peculiar forms. Had I included the West Indies[79] in my first province, the total number of species would have been 42, of which 28 would have been peculiar. The coast of Brazil, even as far south as the Rio Plata, might, also, have been included, for I have not seen from it a single species not included in the above 42 West Indian species. So also, by adding a single species, might the west coast of equatorial Africa. The two coasts of South America and Africa, which face each other within the torrid zone, seem to be remarkably barren in Cirripedes. Europe has several more species than the United States, which is inhabited by only ten species, including even the probably imported _Balanus tintinnabulum_ and _amphitrite_. Of these ten United States species only two are not found in Europe; and both these two range into the West Indies, and as far as the northern shores of South America, and therefore cannot be considered as peculiar to the United States.

[79] The total number of species which I have seen from the West
Indies, is 19 or 20; of these, only 6 are peculiar to it, or 8, if
the United States be likewise included, the other 12 or 14 species
being found in other quarters of the world. Six peculiar species
out of 19 or 20, has not appeared to me sufficient to institute
even a sub-province.

I have formed my single sub-province for the southern extremity of Africa; for although I know of only 11 species from this comparatively short and uniform line of coast, yet I was not able to group these eleven in any of the main provinces: 5 of the species are peculiar, 1 Australian, 3 European and West Indian, and 2 almost universally distributed.

The second province includes the west coast of North and South America, from Tierra del Fuego to Behring's Straits: on this enormously long line of coast, only 22 species are known to exist, but of these no less than 15 are peculiar. Of these 15, four are not found south of the torrid equatorial region, and eight are not known to occur north of this same region; so that this long line of coast might have been divided into two sub-provinces, of which the southern would have been the most peculiar; but as eight species are found both north and south of the equatorial region, I have not made this sub-division. Two of the species occurring on the western coast of North America, seem to represent species found on its eastern coast, and in Europe; thus, _Balanus glandula_ takes the places of _B. crenatus_, and _B. cariosus_ that of _B. balanoides_. Not a single species, excepting a few which are also widely distributed over other parts of the world, is known to be common to the east and west coasts of the two Americas.

The third province is that of the East Indian Archipelago, and includes the Philippines, Borneo, New Guinea, Sumatra, Java, Malacca, and the eastern coast of India. Here we find 37 species, of which 24 are peculiar. I may remark, that I have received no species from Madagascar or the eastern coast of Africa; few from India, or from the coast of China; and I suspect, that on most of these coasts, only few exist. Probably our third province will hereafter be found to include the whole Indian Ocean.

The fourth province is that of Australia, including New Zealand: it has 30 species, of which 21 are peculiar. Had the temperate Australian coasts (_i. e._, those south of the isocryme of 68°) been alone considered, the number of the species would have been probably 25, of which 20 would have been peculiar,--that is, if we admit within the 20, several species which range from the temperate into the torrid zone, but do not extend beyond the Australian shores. Owing to the widely-extended ranges of most Cirripedes, no Arctic or Antarctic provinces can be said to exist.

To recapitulate the above results, bearing in mind that, although the total number of known existing Cirripedes is 147, yet that the habitats of seven are unknown, and that eighteen are excluded owing to their being attached to floating or swimming objects, so that there are only 122 species referred to in the following table:

+--------------------------------------------------+--------+---------+
| | Total | Species |
| | number |confined |
| | of | to the |
| |species.|province.|
+--------------------------------------------------+--------+---------+
|(1.) First, or North Atlantic Province, to lat. }| | |
| 30° N. (If the West Indies had been included}| 31 | 22 |
| the numbers would have been 42 and 28) }| | |
|(2.) Sub-province of South Africa | 11 | 5 |
|(3.) Second province, or West Coast of North and }| | |
| South America }| 22 | 15 |
|(4.) Third province, or East Indian Archipelago | 37 | 24 |
|(5.) Fourth, or Australian province | 30 | 21 |
+--------------------------------------------------+--------+---------+

The least prolific of these provinces contains 22 species, or between 1/5th and 1/6th of the total number of species, and the most prolific between 1/3rd and 1/4th of this same number. In each of these provinces, it is remarkable that the peculiar species are very nearly two thirds of the whole of its inhabitants. These facts, I think, show that the above provinces are natural divisions of the world, as far as their Cirripedial inhabitants are concerned.

As Cirripedes belong to the great class of Crustacea, and as the distribution of the latter has lately been fully discussed by Mr. Dana, it may be worth while briefly to compare my results with his; more especially as they are so very different. I should premise, as perhaps accounting to a certain extent for this difference, that, owing to the wide range of many species, and the almost universal extension of the same genera, my provinces are founded merely on a certain proportion of the species, namely, two thirds, being peculiar or confined to a region of considerable dimensions: whereas, in the case of ordinary Crustaceans, the greater number of the species are distinct even in the sub-provinces, and the provinces are founded mainly on generic differences. Mr. Dana divides the surface of the globe into three great sections, or provinces, the _Africo-Europæan_, the extent of which is shown by its title; the _Occidental_, which includes both the east and west coast of both Americas; and the _Oriental_, including the Indian and Pacific Oceans, with the East Indian Archipelago, and Australia. Thus Mr. Dana entirely separates the Eastern shores of North America from Europe; whereas, as far as their Cirripedial inhabitants are concerned, they are most intimately allied, and form my first or North Atlantic province; and to this, as I have shown, even the West Indies, the coast of Brazil, and equatorial West Africa might have been added. It follows, from this similarity in the Cirripedes on the two sides of the Atlantic, and from their dissimilarity with those on the shores of the Pacific, that the east and west coasts of the two Americas form two quite distinct Cirripedial provinces; though, in the northern half, some connection is shown by a few representative species: on the other hand, Mr. Dana unites both sides of the whole American continent, into his single Occidental province. The South-African province is not brought out by Mr. Dana so prominently, as I have found necessary. Mr. Dana joins the East Indian Archipelago and Australia into his single Oriental province, and makes New Zealand, as a sub-province, apparently as distinct from Australia, as Australia is from the East Indian Archipelago: whereas I find that the Cirripedes of New Zealand clearly belong to Australia; and that the Australian Cirripedes, especially if the temperate shores be alone considered, are as distinct from those of the East Indian Archipelago, as from those of any other quarter of the whole world. I believe that the provinces deduced from the distribution of Cirripedes, accord better with the Molluscan provinces, than with those given by Mr. Dana for the rest of the great class of Crustaceans.

In the following tables, an asterisk means that the species is not found in any other distinct region of the globe. When found in one of the five provinces, a corresponding number, within brackets, is appended, to show in which province or sub-province it has been found.

(1.) FIRST OR NORTH ATLANTIC PROVINCE: _Europe and the Eastern United
States, from the Arctic Regions to 30° north latitude._

Balanus tintinnabulum (1 to 5).
tulipiformis* confined to Europe.
calceolus Europe and India.
{ confined to North America and West
galeatus* { Indies.
spongicola (2) and West Indies (?).
{ confined to Europe, but possibly in the
perforatus* { West Indies.
amphitrite (1 to 5).
eburneus* { confined to North America and West
{ Indies.
improvisus Europe and North America and (3).
porcatus* Europe and North America.
crenatus " " W. Indies and (2).
balanoides* " "
Hameri* " "
Acasta spongites Europe and (2).
cyathus* Madeira and West Indies.
{ confined to Europe, but ranges as far at
Pyrgoma anglicum* { least as the Cape de Verde Islands.
Xenobalanus globicipitis* confined to Europe.
Chthamalus stellatus Europe and North America and (3 and 4).
Pachylasma giganteum* confined to Europe.
Verruca Strömia Europe and Red Sea.
Spengleri* Madeira.
Pœcilasma aurantia* "
crassa* "
Dichelaspis Lowei* "
Oxynaspis celata* "
Alepas minuta* Europe.
parasita* Europe and Atlantic Ocean.
Anelasma squalicola* Europe.
Alcippe lampas* "
Scalpellum vulgare* "
Pollicipes cornucopia* "

Here we have 31 species, of which 22 are not found in any other great region of the world.

(2.) SUB-PROVINCE: _Africa, South of lat. 30°._

Balanus tintinnabulum (1 to 5).
Capensis*
spongicola (1) and West Indies (?).
amphitrite (1 to 5).
crenatus (1) and West Indies.
Acasta spongites (1).
Tetraclita serrata*
rosea (4).
Chthamalus dentatus* also on West Coast of Africa.
Octomeris angulosa*
Scalpellum ornatum*

In this small region we have only 11 species, of which five are peculiar: _Balanus Capensis_ and _Tetraclita serrata_, seem to be representatives of _B. psittacus_ of S. America and of _T. porosa_ of that and several other regions.

(3.) SECOND PROVINCE: _West Coast of South and North America, from
Tierra del Fuego to Behring's Straits._

Balanus tintinnabulum north and south of the equator (1 to 5).
psittacus* south.
vinaceus* "
trigonus north and south (4 and 5).
lævis* "
concavus " (4 and 5).
pœcilus* south.
improvisus south (and north?) (1).
nubilus* north.
glandula north, and Southern Pacific Ocean.
cariosus* north.
Tetraclita porosa north and south (4 and 5 and W. Indies).
Elminius Kingii* south.
Chthamalus stellatus north.
cirratus* south.
scabrosus* "
fissus* north and south.
Hembeli* north.
Verruca lævigata* south.
Pollicipes elegans* south and north.
polymerus* "
Cryptophialus minutus* south.

Here we have on this long line of coast, 22 species, of which 15 are peculiar.

(4.) THIRD PROVINCE: _Indian Archipelago (including the Philippines,
Malacca, Borneo, Sumatra, Java, and New Guinea, and eastern coast
of India)._

Balanus tintinnabulum (1 to 5).
Ajax*
navicula*
stultus and West Indies.
trigonus (3 and 5).
concavus (3 and 5).
amphitrite (1 to 5).
patellaris*
amaryllis (5).
quadrivittatus*
Acasta lævigata and Red Sea.
fenestrata*
purpurata*
sporillus*
Tetraclita porosa (3 and 5) and West Indies.
costata*
vitiata (5).
cœrulescens Pacific Ocean.
radiata (5) and West Indies.
Pyrgoma cancellatum*
grande*
milleporæ*
crenatum*
monticulariæ*
Creusia spinulosa and West Indies.
Chthamalus stellatus (1 and 3).
intertextus*
Chamæsipho scutelliformis*
Octomeris brunnea
Pœcilasma fissa*
eburnea*
Dichelaspis Warwickii*
Ibla Cumingii*
Scalpellum rostratum*
Pollicipes mitella*
Lithotrya Nicobarica*
truncata and Pacific Ocean.

Here we have 37 species, of which 24 are peculiar to this province.

(5.) FOURTH PROVINCE: _Australia (including New Zealand)._

Balanus tintinnabulum (1 to 5).
nigrescens*
decorus*
trigonus (3 and 4).
concavus (3 and 4).
amphitrite (1 to 5).
amaryllis (4).
allium*
vestitus*
imperator*
Acasta sulcata*
glans*
Tetraclita porosa (3 and 4).
rosea (2).
purpurascens*
vitiata (4).
radiata (4) and West Indies.
Elminius plicatus*
simplex*
modestus*
Chthamalus antennatus*
Chamæsipho columna* and Pacific Ocean (?).
Pachylasma aurantiacum*
Catophragmus polymerus*
Alepas tubulosa*
Ibla quadrivalvis*
Scalpellum Peronii*
Pollicipes spinosus*
sertus*
Lithotrya cauta*

Here we have 30 species, of which 21 are peculiar.

_Geological History._

The ancient history of the Balanidæ is a brief one. No secondary species has hitherto been discovered; in my monograph on the fossil Lepadidæ[80] I have shown that the negative evidence in this case is of considerable value, and consequently that there is much reason to doubt whether any member of the family did exist before the Eocene period. The existence of a Cretaceous Verruca is an apparent exception to the rule, as this genus has hitherto always been ranked amongst sessile cirripedes; but Verruca, as we now know, must be placed in a family by itself, quite distinct from the Balanidæ. Balanus is the oldest genus as yet known; it first appeared in Europe and North America, during the deposition of the eocene beds; and was at that time, as far as our information at present serves, represented by very few species. In South America, one species of Balanus abounds in individuals in the ancient Patagonian tertiary formation. I have seen, in the British Museum, specimens, said to have come from the eocene nummulitic beds near the mouth of the Indus, belonging to the second section of the genus. Generally, the extinct forms belong to the last section of this genus, which has the parietes not permeated by pores. During the miocene and pliocene ages, sessile cirripedes abounded. No extinct genus in this family has hitherto been discovered. It is singular, that though the Chthamalinæ approach much more closely than do the Balaninæ to the ancient Lepadidæ, of which so many species have been found fossil even in the older Secondary formations, yet that only one species of one genus of this sub-family has been hitherto found in any deposit; and that species is the still existing _Pachylasma giganteum_, in the modern beds of Sicily. During the epoch of the Glacial deposits in Scandinavia, Scotland, and Canada, the still existing species seem to have abounded; and they attained larger average dimensions than the same species now do on the shores of Great Britain, or even on the shores of the northern United States, where the average size seems larger than on this side of the Atlantic.

[80] Since the note to page 5 of that work was written, I have been
informed that the so-called cretaceous _Tubicinella maxima_ is not
a Cirripede.

Under the genus Balanus, I have given my reasons for never naming species in this large and difficult genus, without examining the opercular valves: it has been owing to this, as it appears to me, proper want of caution, that there are so many nominal species. Thus it is made to appear in catalogues, that the tertiary seas abounded with species of Balanus to an extent now quite unparalleled in any quarter of the world. Bronn,[81] for instance, in his invaluable 'Index Palæontologicus,' gives the names of 35 Balani, found fossil in Europe, and I have not counted those found only in alluvial deposits, as they would certainly be the same as the still living species. Now I know only 11 recent Balani on the shores of all Europe, from the North Pole to lat. 30°; and of these I doubt whether _B. balanoides_ and _improvisus_ have been found fossil. In the Red Crag there is one extinct Balanus: in the Coralline Crag, which seems to have been very favorable to the existence of Cirripedes, there are six species of Balani, of which two are absolutely extinct, and one does not occur in any neighbouring sea: in the Eocene formations the species seem to have been rare, and I have seen only one, and that is an extinct form. Taking these several facts into consideration, and bearing in mind that Cirripedes usually range widely, I do not believe, if all the specimens of Balani hitherto found in the several tertiary formations, from the eocene to the glacial deposits, throughout Europe, were collected together, they would amount to 20 species. I have myself seen, in a recognisable state, only 12 fossil species, of which five are extinct, or not found in any neighbouring sea: I think it probable that three other recent species, viz. _B. tulipiformis_, _perforatus_, and _amphitrite_, may occur in the Mediterranean formations; and this would make 15 species. Therefore in the above estimate of 20 species, five are allowed for species existing in European collections, but not hitherto seen by me; and this, I believe, is a very full allowance. Consequently, even on the supposition that the five species just admitted as possibly existing in cabinets, and that the other five extinct species, which I have seen and examined, have all been previously named by other authors, a supposition excessively improbable, even then there would be 15 superfluous names in Bronn.

[81] To save any other person, interested in fossil Cirripedia,
going through the several works quoted by Bronn, I have given some
remarks on his list of species, in an appendix at the end of the
Balanidæ.

The following short table shows how Cirripedes, including all three Families, were represented in Great Britain, throughout the several TERTIARY STAGES.

A = Living species but found fossil in some tertiary deposit.
B = Mammilliferous crag, and glacial deposits.
C = Red crag.
D = Coralline crag.
E = Eocene.

+----------------------------+---+---+---+---+---+
| Name. | A | B | C | D | E |
+----------------------------+---+---+---+---+---+
| Balanus tintinnabulum | * | | * | | |
| calceolus | * | | | * | |
| spongicola | * | | | * | |
| concavus | * | | * | * | |
| porcatus | * | * | * | | |
| crenatus | * | * | * | * | |
| Hameri | * | * | * | | |
| bisulcatus | | | * | * | |
| dolosus | | * | | | |
| inclusus | | | | * | |
| unguiformis | | | | | * |
| Acasta undulata | | | | * | |
| Pyrgoma anglicum | * | | | * | |
| Coronula barbara | | | * | | |
| Verruca Strömia | * | * | * | * | |
| Scalpellum magnum | | | | * | |
| quadratum | | | | | * |
| Pollicipes reflexus | | | | | * |
+----------------------------+---+---+---+---+---+
| Total, 18, recent and } | | | | | |
| extinct, found fossil in } | 9 | 5 | 8 |10 | 3 |
| Great Britain, in some } | | | | | |
| tertiary deposit } | | | | | |
+----------------------------+---+---+---+---+---+

As affording some standard of comparison by which to compare the number of fossil species, at any period, in relation to the number of species probably existing in the neighbouring seas during the same epoch, I may state that there are now living and propagating on the shores of Great Britain, 18 species belonging to the three Families included in the above table. I have not counted three species, in the genera Alcippe and Conchoderma, which, from the minuteness of their valves, it is hardly possible would be found fossil. On the other hand, I have included in the 18, five species of Lepas, which from floating and being oceanic, are more likely to be cast up on beaches, than to be imbedded in sedimentary deposits; so that 13 would, perhaps, be a safer number, as a standard of comparison. Now in the coralline crag, which seems to have been eminently favorable for the existence and subsequent preservation of Cirripedes, and which has been so well worked, only nine fossil species, as may be seen in the table, have been as yet discovered.

Sub-Family--BALANINÆ.

_Shell with the rostrum having radii, but without alæ; lateral compartments all having alæ on one side and radii on the other side; parietes generally either porose, or longitudinally ribbed on their inner surfaces._

_Mouth with the labrum notched in the middle, not swollen; palpi large, almost touching each other; mandibles generally with the lower teeth laterally double; third pair of cirri with their segments resembling those of the second pair._

_First Section._†

_Scutum and tergum articulated together, or overlapping each other; each branchia composed of a single plicated fold._

_Genera_--Balanus; Acasta; Elminius; Tetraclita; Pyrgoma; Creusia; Chelonobia.

_Second Section._††

_Scutum and tergum (when both are present) not overlapping each other; basis membranous; parietes often deeply folded, with the outer lamina, towards the basis, generally imperfect; each branchia composed of two plicated folds; shell attached to living vertebrata._

_Genera_--Coronula; Platylepas; Tubicinella; Xenobalanus.[82]

[82] At the end of the volume a Synopsis is given, which will serve
as a systematic index for the discovery of generic and specific
names.

The Balanidæ may be divided into two sub-families; namely, the Balaninæ and Chthamalinæ; and, in the former, the genera, as we see, may be very naturally grouped into two sections. The Balaninæ differ from the Chthamalinæ, as far as the shell is concerned, in the rostrum having radii but no alæ, all the lateral compartments having both radii and alæ; on the other hand, in the Chthamalinæ, the rostrum has alæ, and the rostro-lateral compartments radii on both sides, and therefore no alæ. These differences probably arise, as already explained, from the perfect confluence, in the Balaninæ, of the true rostrum with the rostro-lateral compartments. In Chelonobia, belonging to the Balaninæ, we see an intermediate state, with the fusion not quite effected: on the other hand, in one genus amongst the Chthamalinæ, namely, Pachylasma, we must look to the shell at a very early age, to find the rostrum with its alæ, distinct from the rostro-lateral compartments. In Tetraclita, Elminius, and Creusia, the carino-lateral compartments are aborted, or possibly confluent with the lateral compartments, making altogether only four: in Pyrgoma all the compartments are fused together and form a solid ring. The sub-genus Acasta is, in one sense, very natural, as it includes species most closely allied: in another sense it is far from natural, as some of the species can hardly be distinguished from those species of Balanus, which live attached to Gorgoniæ: I almost regret I did not merge the species of Acasta into Balanus. In the Balaninæ generally the parietes are either porose, or are furnished on their internal surfaces with regular ribs, representing the longitudinal parietal septa, which in other species form the tubes or pores; there are, however, many exceptions to this rule in several species of Balanus, in Acasta and Elminius, all of which have the parietes of their shells internally quite smooth, or only irregularly roughened with points.

Looking to the animal's body, in the Balaninæ, the labrum is always notched in the middle, and is never swollen or bullate, for the outer and inner folds of membrane of which it is composed lie close together. The palpi are large, so that their tips almost touch each other. The mandibles, generally, have their lower main teeth laterally double. Of the cirri, the third pair invariably much more closely resembles, in its whole structure, and in its action, the second than the fourth pairs; and it is also generally separated by a small interval from the fourth pair.

I have already under the Family sufficiently entered on the relations of the Balaninæ to the Chthamalinæ, and of the genera, one to the other, so that I need not here add anything.

I can point out no difference in habits or geographical distribution between the Balaninæ and Chthamalinæ.

1. _Genus_--BALANUS, Auct.[83]

[83] The name Balanus was used, almost as at present, by Lister and
Hill, before the introduction of the binomial system. Since that
period the first two authors, as far as I know, who used this name,
were Da Costa, in his 'Hist. Nat. Test. Brit.,' in 1778; and Bock,
in the 'Naturforscher,' for the same year; Bock, however, applied
it to a Chelonobia.

CONOPEA (pars generis). _Say._ Journal Nat. Sc. Philadelphia, vol.
ii, part ii, 1822.

MESSULA (do.). _Leach._ Zoological Journal, vol. ii, 1825.

CHIRONA (do.). _J. E. Gray._ Philosoph. Transacts., 1835, p. 37.

_Compartments six; basis calcareous or membranous; opercular valves sub-triangular._

_Distribution._--Mundane: in the warmer seas.

_General appearance._--The shape of the shell in the different species varies from depressed conical to cylindrical; the latter form being generally assumed when specimens are crowded together; but some species, as _B. balanoides_, _crenatus_, and _lævis_, seem more subject than others to be thus affected. The colour is either white, generally tinted by the yellowish or brownish epidermis, or any colour intermediate between bright pink and rich blue, purple being the prevailing tint. The persistence of the so-called epidermis is very different in different species, being even sometimes highly variable in the same species. The surface is either smooth or more commonly folded longitudinally, or sharply ribbed. The orifice differs in form from diamond-shape to trigonal; the carinal end, owing to the shape of the carina, being always sharper or narrower than the rostral end. The size of the orifice, in proportion to the shell, varies accordingly as the latter is more or less conical or cylindrical. The orifice is either entire or more or less deeply toothed, in proportion to the degree of obliquity of the summits of the radii and alæ. The radii almost always have smoother surfaces than the parietes. In some few species the radii are not developed, the sutures being marked only by fissure-like lines; in others they are very narrow, and in this case their upper margins are generally rounded and smooth, instead of being straight and jagged. The carino-lateral compartments are usually much narrower than the lateral compartments, occasionally in an extreme degree, as in _B. allium_. The shell is generally strong, sometimes to a wonderful degree; but the strength and thickness vary in the individuals of some of the species. By the action of hot caustic potash, the compartments in several species, such as _B. Hameri_ and _crenatus_, separate on a touch; in others, they adhere so strongly as to prove that the sutures must be calcified together. In this genus we have the largest known sessile cirripede, viz., the _B. psittacus_, and on the other hand many small species; but it is very difficult, except in well-known species, to ascertain the average or even the maximum dimensions.

_Scutum._--This valve is almost triangular, with the basi-tergal corner more or less rounded off. The prominent lines of growth are sometimes crossed by longitudinal striæ. Internally, the articular ridge projects to a very different degree in the different species; its lower end is sometimes (as in _B. lævis_, Pl. 4, fig. 2 _c_) produced downwards as a small, sharp, free style; there is always an articular furrow receiving the inflected margin of the tergum. There is always an impression left by the attachment of the adductor scutorum muscle; and often its lower side is bounded more or less closely by a sharp adductor ridge, running some way down the valve; this ridge is occasionally almost confluent, in its upper part, with the articular ridge, and in this case sometimes it forms, together with the inflected tergal margin, a large tubular cavity, running up, as in _B. psittacus_ (Pl. 2, fig. 3 _c_), almost to the apex of the valve. Almost invariably there is a slight pit or depression for the lateral depressor muscle; sometimes within the depression there is a little ridge, as in _B. perforatus_ and _nubilus_ (Pl. 4, fig. 3 _a_, and Pl. 6, fig. 2 _a_); and in the case of _B. vestitus_, _flosculus_, and _imperator_ (Pl. 8, figs. 3 _a_, 4 _a_), there are regular crests for this same purpose. The rostral depressor muscle is usually attached in a small pit formed by the folding over of the lower part of the occludent margin: in _B. imperator_ (Pl. 8, fig. 4 _a_) there are regular crests for its attachment, and traces of them may be discovered in _B. vestitus_.

_Tergum._--This valve is more nearly triangular than any other shape, with the spur more or less prominent. The apex generally projects a little above the level of the scutum; in some species it consists of a triangular and solid, in others (Pl. 2, fig. 3 _b_) of an almost cylindrical, extremely sharp, inwardly curved, and very prominent beak. This beak is generally purple; it is sometimes hollow, and occupied by a thread of corium. Its formation, and the apparent sliding up of the whole tergum, so as to project above the scutum, has been described under the family. From an account given to me by a person who kept _B. porcatus_ alive, the beaks appear to be used, when the operculum is touched, as an organ of defence,--the animal striking with them. The tergal margin is more or less inflected; and the carinal margin is convex in different degrees, and, in some species, is added to by upturned zones of growth. The basal margin either forms a nearly straight line on opposite sides of the spur, or more commonly slopes towards it in various manners. The spur, or basal projection, is rarely placed in the middle of the basal margin, generally near, sometimes close to the basi-scutal angle; it varies much in length and breadth, and is sometimes even half the width of the valve. The surface of the valve is almost always more or less depressed, sometimes so much as to form a deepish furrow, the "longitudinal furrow," which extends from the apex to the extremity of the spur. When the furrow is deep, its sides, as the specimen grows old, almost always become folded inwards, so as to touch, and then the furrow becomes converted into a closed fissure: in this latter case the folded sides generally form a central crest on the spur. Internally, in the middle of the upper part of the valve, the articular ridge is more or less prominent, forming the carinal margin of the articular furrow, in which the articular ridge of the scutum is lodged; occasionally, however, this articular ridge can hardly be said to exist. In most species the tergal depressor muscle is attached to sharp crests on the basi-carinal corner of the valve, but these are almost obliterated in other species.

_Compartments._--The external appearance of the shell has already been described. In the most typical species, the parietes consist of an outer and inner lamina, separated by strong longitudinal septa; these septa are denticulated on both sides at their bases, but only close to the inner lamina; in fact the inner lamina is apparently formed by the union, thickening, and production, of some of the denticuli. As it is not the innermost of the denticuli on the basal edges of the longitudinal septa, which thus become united into a solid layer, the longitudinal septa form slightly projecting, longitudinal ribs on the inner lamina. These internal ribs are longitudinally striated; in old specimens they often become obliterated, especially in the upper part of the shell. The parietal tubes or pores (occupied by threads of corium) are generally square and large; but in _B. Ajax_ they are very small, and in _B. glandula_ often extremely minute. In the upper part of the shell, and sometimes low down, they are generally crossed by thin, transverse, calcareous septa: in some species, as in _B. perforatus_, and in some varieties of _B. amphitrite_, the upper ends of the tubes are filled up solidly with shell. In some varieties of _B. crenatus_ and of _amphitrite_, the longitudinal septa, near the outer lamina, divide, thus giving rise to a very imperfect row of outer short tubes. In _B. vinaceus_ (Pl. 2, fig. 7 _d_) the inner lamina is cancellated instead of being solid, which is caused by the basal denticuli of the longitudinal septa being simply united together by their ends and crossed by transverse septa, instead of being consolidated into a mass. In several species, as in _B. Hameri_, the walls consist only of the outer lamina with longitudinal ribs, no inner lamina having been formed; the ribs here evidently answer to the longitudinal septa in the foregoing species. In _B. flosculus_ and _imperator_ the walls are solid, their basal margins being formed of irregular, elongated points, and little ridges (Pl. 8, fig. 4 _c_), which apparently prefigure the more regular longitudinal ribs or septa. In _B. balanoides_ the walls are generally either nearly smooth and solid, or irregularly cancellated; in _B. cariosus_ (Pl. 7, fig. 3 _b_) two or three rows of short irregular tubes are formed by unequally branching septa, almost as in the genus _Tetraclita_.

The _Radii_, in all the species, are constructed essentially on the same plan as the parietes; thus, in the typical forms, there is an outer and inner lamina, with septa, which, near the inner lamina, are furnished with denticuli on both sides; hence the radii are permeated by pores or tubes, like the parietes; but this holds good only in the first section of the genus, for, in the other species, the tubes are filled up quite solidly. The denticuli on the septa often occur only on one side, or disappear altogether; and, lastly, the septa themselves often appear merely like little teeth, or disappear altogether as in _B. Hameri_, or occur only near the bases of the radii, as in _B. amaryllis_. A slight furrow in the compartment, against which each radius abuts, is generally marked by the septa and their denticuli. In regard to the _alæ_, their lateral or sutural edges are either thin and smooth, or, more commonly, finely crenated or ribbed. The little transverse crenations are homologous with the septa in the radii and parietes. The edges of the alæ are usually received in a furrow. The diametric growth of the shell is effected by the growth of the radii and alæ, and chiefly by that of the former. The sutural and lateral edges of both radii and alæ are added to, either quite up to their summits, or only low down, and during the continued growth of the shell, lower and lower down; in accordance with this difference in growth, the summits of the radii and alæ become either very oblique, or they extend parallel to the basis, that is, from tip to tip of the adjoining compartments. When the radii and alæ are added to, as is most usual, above the level of the opercular membrane, and therefore above the sack, ribbons of corium run up the sutures from the sack, higher or lower, according to the height to which, in the different species, the edges of the radii and alæ continue to be added to. The obliquity of the summits of the radii and alæ varies, in some cases, in the same species. It often happens that when the summits of the radii are very oblique, the summits of the alæ are but little so; and the converse; both, however, are often either equally oblique, or both have square summits. The _sheath_ extends either one third or more than half down the shell; its basal margin often (Pl. 25, fig. 1, K′) freely depends or overhangs the inner lamina of the walls.

_Basis._--In typical species the basis is calcareous, and consists of an upper and lower lamina, separated by radiating septa, forming pores. In the same manner as the septa of the parietes sometimes, though rarely, become irregularly divided near the outer lamina, forming outer pores, so it is, but in a much more marked degree, with the basis. The basis in such cases becomes extremely thick, and consists of an upper, thin lamina, with the regular radiating septa and pores, and of an underlying, thick, cancellated mass, which seems wholly to result from the dividing and sub-dividing of the septa. The basal radiating pores, like the parietal pores, are closed at intervals by calcareous transverse septa. The basal points of the parietal septa enter the orifices of the basal pores, and the threads of corium pass into the latter, between the denticuli of the parietal septa. In some species, as in _B. crenatus_ and _Hameri_, the basis is perfectly solid, the upper lamina being absent, just as in some species, the internal lamina of the parietes is absent. In _B. flosculus_ the basis is calcareous, but consists of so excessively thin a film as hardly to be distinguished: it presents, moreover, as also is the case with _B. imperator_, a beaded structure. Again, in some few species, as in _B. balanoides_, the basis is simply membranous. When the basis is thin, it is always flat, and is closely moulded to the irregular surface of attachment; and in this case, when specimens are crowded together, their elongation is effected exclusively by the growth of the walls; but, when the basis is thick, it sometimes becomes, in crowded groups, deeply, but irregularly, cup-formed, or cylindrical, as in _B. psittacus_ and _perforatus_. In _B. allium_, however, which inhabits massive corals, the basis is as regularly concave or cup-formed as in the genus Pyrgoma: in _B. calceolus_ and its allies, and in some varieties of the fossil _B. inclusus_, the basis is boat-formed, with its lower surface deeply grooved longitudinally from clasping the stem of the Gorgonia or other zoophyte, to which it was attached. In certain varieties of _B. lævis_ it is very remarkable that the deeply cup-formed basis becomes, owing apparently to the whole shell having grown too deep for the animal, half-filled up with irregular, calcareous, transverse plates (Pl. 4, fig. 2 _a_), resting one upon the other by irregular points or pillars. The cementing apparatus has been sufficiently described under the Family.

_Mouth._--The _labrum_ is always notched; sometimes it has no teeth, but generally there are three on each side; in _B. balanoides_ there are five or six on each side; and in _B. improvisus_ and _eburneus_ there is a whole row of teeth (Pl. 26, fig. 2, _e′_), graduated in size, on each side of the notch. The _palpi_ are large, with their apices nearly touching, and furnished with long spines. The _mandibles_ have, as it appears, normally, five teeth, but the two lower teeth are always small and often rudimentary, and almost confluent with the inferior, sometimes spinose angle. The _maxillæ_ have either a simple edge, or a notch under the pair of large upper spines, or the lower part forms (Pl. 26, fig. 7) a step-formed projection: there are generally two lower spines, placed singly or not in pairs, larger than the others, with the exception of the uppermost pair. The _outer maxillæ_ are, on their inner faces, obscurely divided into two lobes.

_Cirri._--The rami of the first pair are unequal, the shorter one sometimes not being more than half the length of the other ramus: the segments of the shorter ramus are broad, and are, together with the lower segments of the longer ramus, thickly clothed with spines; in some species, as in _B. perforatus_, the anterior surfaces of the segments, more especially of the shorter ramus, and of both rami of the second pair are produced (Pl. 29, fig. 4), so as sometimes to form very remarkable projections. The segments of the second and third pairs are always thickly clothed with spines, as also are their pedicels. The third pair is rather longer than the second; but in _B. vestitus_ and _imperator_ it is much longer, and is otherwise somewhat different. The dorsal and basal margin of the pedicel of the third pair, in some of the species, as in _B. tintinnabulum_, is produced backwards on the thorax, and forms a membranous plate fringed with fine spines. The three posterior and longer pairs of cirri have from three to rarely eight or ten pairs of long spines on each segment, with generally one or two minute spines in the middle between each pair: their pedicels have a regular double row of spines.

The _penis_ is long and hairy: in most of the species there is, at its dorsal basis, a small, sharp, flattened, imperforate projection; first observed by Poli: but this is sometimes absent, as in _B. crenatus_, though present in the closely allied _B. balanoides_; and its presence is variable in _B. tintinnabulum_. All the species have large plicated _branchiæ_. The base of the _sack_ in several species is furnished with inwardly projecting filamentary appendages. In _B. perforatus_, _crenatus_, and _improvisus_, and I believe in other species, the upper part of the _stomach_ is furnished with a circle of branching cæca.

_On the variation of the species_; _their arrangement and affinities_; _value of the characters used_; _changes during growth_.--Owing to the great variation in external characters, to which almost all the species are subject, and likewise to the genus being a very natural one, that is, to the species following each other in close and natural order, it is not easy to exaggerate the difficulty of identifying the species, except by a deliberate examination of the internal and external structure of each individual specimen. Every one who has collected sessile cirripedes must have perceived to what an extent their shape depends on their position and grouping. The surface of attachment has a great effect on that of the shell; for as the walls are added to at their bases, every portion has at one time been in close contact with the supporting surface; thus I have seen a strongly-ribbed species (_B. porcatus_) and a nearly smooth species (_B. crenatus_) closely resembling each other, and both having a peculiar appearance, owing to their having been attached to a pecten. Dr. Gray has pointed out to me specimens of _B. patellaris_, curiously pitted like the wood to which they had adhered; and numberless other instances might be added. Quite independently of the effect produced by the surface of attachment, the degree to which the longitudinal folds and ribs are developed on the parietes, is variable in most of the species, as in _B. tintinnabulum_, _vestitus_, and even in _B. porcatus_; the presence or entire absence of these ribs often surprisingly alters the whole aspect of the shell. The persistence of the so-called epidermis is in some degree variable; and in _B. lævis_ we have groups of specimens absolutely naked, and others uniformly clothed with a brown membrane. Again, some species in certain localities are all subject to the disintegration of the entire outer lamina of the walls; and in such cases (as with _B. perforatus_) there is not the smallest resemblance between the corroded and perfect specimens. The size of the orifice, and consequently of the operculum, compared with the shell itself, varies accordingly as the shell is more or less conical or cylindrical; in the latter case, the summits of the radii are generally more oblique and the aperture consequently more deeply toothed than in the more conical varieties. Size is a serviceable character in some cases, but very many specimens are required to ascertain the average or maximum size of each species, for there is no method of distinguishing a half-grown from a full-grown specimen; and I believe, as long as the individual lives, so long does it go on moulting and growing. _Colour_ is of very considerable service; though the precise tint varies greatly in almost every species; and what is a far more serious evil, the majority of the species have their white or nearly white varieties, the latter sometimes as numerous as the coloured ones: in _B. perforatus_, _lævis_, _flosculus_, _amphitrite_, and in several other species, the common white varieties are eminently deceptive.