Chapter XXII: Introduction (18)

_Appearance and Structure of Shell._--Shell white, or with a tinge of
purple; nearly flat, with moderately prominent, narrow, approximate
ridges, radiating from the orifice, which is oval, rather narrow,
and not very small. In the largest specimens the ridges are less
prominent than in the figure given. The walls are thick, and not at
all porose: the sheath extends down almost to the base of the shell,
and its lower edge is closely attached to the walls: on each side,
towards the carina, there is a trace of a suture, and the lines of
growth on the sheath are here a little upturned. The basis is deeply
imbedded and internally furrowed; the calcareous layer forming it, is
thin. The largest specimen was .4 of an inch in the longer diameter.

The _Scuta_ and _Terga_ (fig. 7 _b_, 7 _c_), in all the specimens
which I have seen, are calcified together, with no trace of a
suture. There is, however, a slight furrow, which, I believe, marks
the normal line of separation between the two valves; and in the
following description this is assumed to be the case. The _Scutum_
has the adductor ridge greatly developed, so as to project below
the ordinary basal margin to a distance as great as the height of
the valve. At the rostral end, this adductor ridge or plate is
produced into a point; and at the tergal end, it is blended with the
articular ridge, and united to the inner face of the tergum. That
portion of the scutum which corresponds with the valve in ordinary
cases, and alone is externally visible as long as the operculum is
united by the opercular membrane to the sheath, is narrow, with the
basal margin considerably hollowed out: the occludent edge is formed
into thick teeth. The _Tergum_ is elongated, rather exceeding in
length the scutum, the latter being measured from the apex to the
rostral projection of the adductor plate. The surface of the valve is
depressed in the line of the spur, with the basal end of the latter
bluntly pointed. A very slight flexure (fig. 7 _c_) on the basal
margin indicates where we may believe the spur to commence, showing
that it rather exceeds in length the whole upper part of the valve.
The lines of growth obscurely indicate a tendency to the formation of
a slight "occludent ledge" along the carinal margin. Traces are just
visible of crests for the attachment of the tergal depressor muscles.

5. PYRGOMA GRANDE. Pl. 13, fig. 1 _a_-1 _d_.

NOBIA GRANDIS. _G. B. Sowerby, junr._ (sine descript.) Conchological
Manual, fig. 29, 1839.[118]

CREUSIA GRANDIS. _Chenu._ Illust. Conch., Tab. 1, fig. 2 _a_, sed
non, fig. 2.

[118] It is quite possible that this may be the _Balanus
duploconus_ of Lamarck, but with such a character as the following,
who can recognise a species? "_B. testæ parte supremâ univalvi,
indivisâ, convexâ: inferiore turbinatâ, non clausâ: aperturâ
ellipticâ._ L'exemplaire est sans opercule et incomplet."

_Shell moderately convex, nearly smooth: scutum and tergum calcified together without any suture: scutum furnished with a small occludent ledge, with the adductor ridge descending below the basal margin: tergum square without a spur._

_Hab._--Singapore and East Indian Archipelago; Mus. Brit., Cuming,
Stutchbury; imbedded in two kinds of coral.

_Appearance and Structure of Shell and Basis._--The shell is conical,
though to a variable degree, and sometimes is much depressed. The
surface is smooth, with only traces of narrow approximate ridges.
The colour is white, often with a tinge of dark purple. The orifice
is oval, and moderately large. The shell and a small portion of
the basis usually stand exserted above the coral. The walls are
of variable thickness; when thick, the pores, by which they are
permeated, are but little apparent; sometimes there is more than a
single row of pores. The points of the septa on the basal edge of the
shell are small. The internal surface of the shell is smooth. The
sheath is closely attached to the walls, and descends nearly to the
basis; on each side its lines of growth are slightly upturned. The
basis is deeply cup-formed or cylindrical, and in section oval like
the shell; it penetrates the coral to a very remarkable depth,--in
one instance to three inches. The shelly layer forming it, is thin,
finely furrowed, and not permeated by pores. This is the largest
species in the genus; one specimen was three quarters of an inch in
its longer diameter, and above three inches in length.

The _Scuta_ and _Terga_ are calcified together, without any trace
of a suture; the line of junction can be inferred only from the
analogy of _P. conjugatum_, in which species the valves have a more
normal character, and are separated by a slight furrow. It may be
seen in the figure (Pl. 13, fig. 1 _d_) of the right and left hand
opercular valves, viewed from vertically above in their proper
relative positions, how abnormal their appearance is, which is partly
caused by the spinose occludent ledges, presently to be described,
but chiefly from the carinal margins of the two terga not being
straight and parallel, as in all other cirripedes, and therefore not
meeting each other, as is usual. In other genera, only the upper
part of the carinal margin of the two terga can be opened for the
exsertion of the cirri, the lower portion being united by membrane;
but here, I have little doubt, from the position of attachment of
the adductor muscle (fig. 1 _c_), so close to and almost on the
terga, that the whole length of the carinal margin of the two terga
is free or disunited for the protrusion of the cirri. This opening
between the two terga evidently cannot be closed, but is probably
filled up, and the animal thus protected, by the dorsal surfaces of
the curled-up cirri; such, I believe, being likewise the case with
some pedunculated cirripedes, as with Conchoderma. The _scutum_ has
a large adductor plate, which extends some little way (namely, about
one quarter of the height of the valve), below the ordinary basal
margin. This latter margin is slightly sinuous, and a little hollowed
out towards the tergal corner of the valve. I believe that the ridge,
which runs down to the basi-scutal corner of the tergum, though
appearing to be part of the scutum, really belongs to the tergum.
The adductor scutorum plate is not, as in the last two species,
produced into a point at the rostral angle; at the tergal end it
blends into the under surface of the tergum. The occludent margin
is coarsely toothed. Rather on the under side of this margin and in
the upper part, there is a narrow occludent ledge, which extends up
beyond the apex of the valve, and thence runs a little way along the
carinal margin of the tergum. This ledge is thickly clothed with
strong, yellowish-brown spines, and hence appears like a brush. It
is remarkable that the cavity for the adductor scutorum muscle is
situated almost on the tergum.

The _tergum_ is of large size, and nearly square; it is, in
appearance, separated from the scutum by a ridge running up to the
apex. The basal margin forms a right angle with the carinal margin,
along which latter margin the lines of growth are upturned, and blend
into the occludent ledge, which is common to the two valves. There
cannot be said to exist any spur, the whole basal margin being almost
straight; nevertheless, on close examination, the ridge which in
appearance separate the scutum and tergum, may, I think, be safely
considered as one side of the spur (which, it should be remembered,
has in all ordinary cases a longitudinal furrow or depression), and
the other side of the spur is, apparently, very feebly indicated by a
slight flexure in the middle of the basal margin. Hence, if the spur
had been developed, it would probably have been half as wide as the
valve. There are no crests for the tergal depressor muscles.

_Affinities._--The present species, with the last two, form an
interesting series. _Pyrgoma grande_ and _conjugatum_, however,
are more closely allied to each other than to _P. cancellatum_. In
the scutum, the whole valve is least elongated, with the adductor
plate least developed, in _P. grande_, and most elongated, with
the adductor plate most developed, in _P. cancellatum_. In the
outline of the tergum the range of shape is quite remarkable; in _P.
conjugatum_, which stands between the other two species, the spur is
rather long, whereas in _P. grande_ there is no spur at all--a very
unusual circumstance--and in _P. cancellatum_, at the other end of
this short series, the spur attains a length wholly unparalleled in
any other cirripede.

6. PYRGOMA MILLEPORÆ.[119] Pl. 13, fig. 2 _a_-2 _f_.

[119] From external aspect I suspect this species to be the
_Creusia madreporarum_, Leach (?), as given in Chenu, 'Illust.
Conch.,' Tab. 1, fig. 6. But I feel sure that Leach has nowhere
published this name; and it may be observed that Chenu gives it
with a mark of doubt. The shell in its imbedded state is only
figured; the opercular valves are not given; and no descriptive
details are added. Under these circumstances I have not adopted
this name; I have, perhaps, been in some degree influenced by the
fact that this species, judging from the many specimens examined by
me, is never imbedded in madrepores, but exclusively in millepores,
so that Chenu or Leach's specific name of _Madreporarum_ is
singularly inappropriate.

_Shell with the orifice narrowly ovate: sheath dark purple: Scutum much elongated: tergum triangular, convex, without a spur._

_Hab._--Philippine Archipelago (Mindoro Island), Mus. Cuming. Mus.
Brit., Stutchbury, &c. Imbedded in _Millepora complanata_, sometimes
associated with _Balanus Ajax_.

_Appearance and Structure of Shell._--Shell oval, flat, coloured pale
dull purple, or white, with slight and narrow ridges, radiating, from
the orifice, which is not quite central, but is placed rather nearer
to the carinal than to the rostral end of the shell. The orifice is
small and narrow; the carinal end being rounded, and the rostral end
narrower and sharper,--this being the exact reverse of the usual
shape of the orifice in the Balanidæ. The walls are thick, and are
formed of large square tubes. The internal surface of the shell is
smooth. The sheath (fig. 2 _b_) is much more elongated than the
shell, for at the rostral end it extends to the basal margin, and at
the carinal end nearly to it, whereas on the two sides it is some way
distant from the base. The orifice is considerably out of the centre
of the sheath, being placed nearest to the carinal end. The sheath
has its lower edge slightly prominent or free; the lines of growth
are neither turned up nor down on the two sides, as in the last three
species. When nearly full grown the sheath is coloured dark purple,
but when young it is white, hence the upper part is white, surrounded
by an oblong purple ring, and this is surrounded by the white shell.
The basal cup is deep, and internally nearly smooth. The largest
specimens were .3 of an inch in their longer diameter. Great masses
of the Millepora are absolutely studded with this Pyrgoma, with
usually more specimens on one side of the plate or branch than on the
other. They stand in approximately parallel positions, the broad or
carinal ends of the orifices pointing upwards.

_Scuta._--The scuta and terga are closely united, and _are_ often
(perhaps always) in some slight, though variable degree, calcified
together; and hence they often break, rather than separate, at the
line of articulation. The external fissure or line of junction
between them (fig. 2 _c_) is oblique to the longer axis of the
scutum; in the uppermost part of the valve it is sometimes almost
obliterated. The two valves together are nearly as long as the
sheath, and consequently much longer than the orifice of the shell.
The scutum is much elongated, being fully four times as broad as
high. The valve narrows towards the rostral end, but in a variable
degree: the basal margin is hollowed out a little (but to a variable
amount), close to the basi-tergal corner. Along the occludent margin
a slip of the valve, widening downwards, is a little bent inwards,
and this inflected portion is separated from the rest of the surface
by a slightly angular ridge, running from the apex to near the
rostral angle. Internally, at the basi-tergal corner, a slight ridge,
parallel and close to the basal margin (and which can be seen only
when the basal edge of the valve is held upwards), represents the
adductor ridge, which we have seen so largely developed in the last
three species, and shall again see in _P. crenatum_. The articular
ridge (fig. 2 _d_) is extremely prominent, consisting of a more or
less rectangular shoulder.

_Terga_: these are rather small compared to the scuta: they are
triangular and much arched: there is no trace of a spur. Internally
(fig. 2 _f_), the articular ridge is central: there are some vestiges
of crests for the depressor muscle.

_Affinities._--This well-marked species, in the tendency of the
opercular valves to be soldered together, and in the remarkable
absence of a trace of a spur to the tergum, is allied to _P. grande_,
but it is more closely allied to the three following species.

7. PYRGOMA DENTATUM. Pl. 13, fig. 3 _a_-3 _g_.

_Scutum much elongated, with a tooth-like articular projection: tergum convex, irregularly triangular, sometimes with an imperfect spur, and on the internal surface with an inwardly projecting tooth; scutum and tergum furnished with an occludent ledge._

_Var._ (1), 3 _c_, 3 _g_: _tergum, with a sharp internal tooth,
projecting rectangularly inwards_.

_Var._ (2), 3 _d_: _tergum, with a broad blunt internal tooth,
depending beneath the spur-like portion of the valve_.

_Var._ (3), 3 _f_: _tergum, with the basi-carinal end of the valve
truncated, with a small blunt internal tooth projecting rectangularly
inwards_.

_Hab._--Red Sea; Mus. Brit. and Cuming. Also associated with _Pyrgoma
crenatum_, and attached to _Meandrina spongiosa_.[120]

[120] Mr. Dana informs me that he believes that this coral comes
from the West Indies; though the specimens originally described
by him had no label. If this be so, both _Pyrgoma dentatum_ and
_crenatum_ have very wide ranges.

_Appearance and Structure of Shell._--Shell nearly flat, oval,
white or pink, with rather distant prominent ridges radiating from
the moderately large (for the genus) oval orifice. The ridges are
often obscured, and apparently sometimes almost obliterated by the
encrusting coral. Shell permeated near the outer lamina by short
imperfect pores: internal surface smooth: sheath inconspicuous,
descending rather more than half way down the walls; lower edge
closely attached to the walls. Basis deep. Diameter of largest
specimen .3 of an inch.

_Scuta_: these are elongated, but to a very variable degree, some
specimens being quite three times, and some barely twice as broad
as high. I observed this same variability in two sets of specimens,
differing, as we shall presently see, in the form of their terga:
it depends in part, but not wholly, on the varying width of the
occludent ledge, which is sometimes only a fourth, and sometimes
half as high as the rest of the valve. This is the first species
in the genus in which the occludent ledge--a structure peculiar
to the genus--has been amply developed. The basal margin of the
valve is slightly sinuous, and a very little hollowed out near the
basi-tergal corner; it is also very slightly reflexed, the reflexed
portion being separated from the upper part of the valve by a very
slight depression or even furrow. I notice this slightly reflexed
portion, simply as indicating a well marked feature in the basal
margin of the following closely allied species. Internally (3 _g_),
the adductor ridge is thick and slightly prominent, but does not
descend beneath the basal margin: it blends into the articular ridge,
which here projects in a remarkable manner and degree (3 _b_, 3 _e_,
3 _g_), like a rounded tooth. This tooth is in part a development of
the occludent ledge; it varies much in shape. The line of junction
between the scutum and tergum is nearly straight, and nearly at right
angles to their longer axes. In some specimens the scuta and terga
are partially calcified together.

_Terga_: in three sets of specimens the terga differed considerably,
but as in every other respect there was the closest resemblance,
I do not doubt that these are merely varieties. In all three, the
valve is rather small, irregularly sub-triangular in shape, and
externally somewhat convex; in all three, there is an occludent
ledge, of variable width as in the scuta; and in all three, there is
an internal tooth-like projection, of variable form, unlike anything
I have seen in any other cirripede. In the _first variety_ (Pl. 13,
fig. 3 _c_, 3 _g_), the basi-carinal corner of the valve is bluntly
pointed, and a slight flexure separates this portion of the valve
from the other and scutal half, which latter thus exhibits some
tendency to be converted into a spur: on the internal surface (3
_g_) of this spur-like portion of the valve, there is a rather long,
sharp tooth, which projects rectangularly inwards; it is flattened
in a plane at right angles to the longer axis of the scutum and
tergum together: it cannot be seen from the outside. In the _second
variety_, the shape of the valve is not very different (Pl. 13, fig.
3_d_), excepting that the flexure, separating the basi-carinal corner
of the valve from the spur, is deeper; but on the internal face of
the spur, the tooth is far broader than in the first variety, and
is flattened quite differently, viz., in a plane nearly parallel to
the surface of the valve, and instead of projecting rectangularly
inwards, it depends beneath the basal edge of the so-called spur, and
can be seen from the outside. In the _third variety_ (fig. 3 _f_),
the whole carinal end of the valve is cut off, and there can hardly
be said to be any trace of a spur, yet a slight furrow apparently
marks the line of separation between the basi-carinal portion of
the valve, here become very narrow, and the broad, irregular part,
which would have formed the spur had such been developed: on the
internal surface of the latter portion of the valve, there is a very
small, blunt, slightly flattened tooth, projecting inwards, and more
resembling that in the first than that in the second variety.

_Affinities_: under _P. crenatum_ I will point out the diagnosis and
relationship between this and that species.

8. PYRGOMA CRENATUM. Pl. 13, fig. 4 _a_, 4 _b_.

PYRGOMA CRENATUM. _G. B. Sowerby._ Genera of Recent and Fossil
Shells, (No. 218, Sept. 1823), fig. 1 to 6.

_Scutum much elongated, with the adductor ridge descending below the reflexed basal margin: tergum with a broad depressed spur: scutum and tergum furnished with a wide occludent ledge._

_Hab._--Philippine Archipelago; Singapore; Mus. Brit., Cuming,
Stutchbury; sometimes associated with _Creusia spinulosa_.

_Appearance and Structure of Shell._--Shell not distinguishable
from that of _P. dentatum_; nearly flat, oval, white, sometimes
pale pink, with rather distant prominent ridges radiating from the
moderately large oval orifice. Shell solid, or permeated near the
outer lamina by short imperfect pores: internal surface smooth. The
sheath descends nearly to the base of the walls; it is but little
conspicuous, and its lower edge is closely attached to the inner
surface of the shell. Basis deep. Diameter of largest specimen under
.3 of an inch.

_Scuta_: in this species the scutum is more abnormal than in any
other Cirripede in the whole family: this is owing both to the
adductor ridge descending far beneath the basal margin, and to the
great development of the occludent ledge; hence the middle and very
narrow portion of the valve alone answers to the scutum, as seen in
other genera. The whole valve, including the adductor ridge and the
occludent ledge, is narrow, being more than twice as broad as high;
but the proportional width varies, owing chiefly (as in the last
species) to the varying width of the occludent ledge. This ledge
commences a little way from the rostral point of the valve, and
gradually widening, extends to the apex, where it is either as high
or twice as high as the rest of the valve. It is articulated by a
convex surface, and by a hollow on its under side with the occludent
ledge of tergum. The scutum cannot be said to have any tergal margin;
without, indeed, the articular surface of the occludent ledge be
thus called. The basal margin is curved, and considerably reflexed,
of which peculiarity we have seen a vestige in the last species:
the reflexion is not well shown, owing to the foreshortening of the
reflexed edge, in fig. 4 _a_: this reflexed edge not being shown,
causes the lines of growth to appear as if they ran more transversely
to the longer axis of the valve, than they really do; for they run
nearly as in the scutum (fig. 3 _e_) of _P. dentatum_. The direction
of these lines of growth is of importance, for they show that the
elongation of the scuta is due to an inordinate increase in their
breadth, as compared to the same valves in ordinary species. The
adductor ridge, having a sinuous margin, runs from near the apex to
near the rostral angle: it descends below the basal margin about as
far as the height of the true valve, excluding the occludent ledge.
Of course this adductor ridge or plate lies beneath the membrane
connecting the opercular valves with the sheath, and is concealed by
it, as long as these valves remain within the shell. The edge of the
occludent ledge is straight, but the edge of the middle portion of
the valve, that is of the true valve, is much bowed.

_Terga_ (fig. 4 _b_): these are of so irregular a shape that they
can hardly be described; they may, however, be said to consist of
two portions joined together, of which the lower portion is a little
elongated transversely, of somewhat variable shape, with part of its
surface considerably depressed (compared with the rest of the valve),
sometimes being even concave: this concave portion apparently answers
to the spur in other cirripedes. At the scutal corner of the valve
there is a shoulder (perhaps answering to the inflected scutal margin
in an ordinary tergum), which locks into a hollow on the under side
of the occludent ledge of the scutum. The internal surface of the
so-called spur is rounded and convex. The upper part of the tergum
is in main part formed by a great occludent ledge; but this, on its
lower side, is bordered by a narrow irregular slip, which, as shown
by the lines of growth, represents the whole of the ordinary valve,
excepting, of course, the spur already described. The occludent
ledges of both valves support some fine spines.

_Affinities._--Observing how extraordinarily the terga varied in _P.
dentatum_, and that the shells were identical in that and the present
species, it occurred to me at first that they might, perhaps, be both
extreme varieties of one form: but in the scutum of _P. crenatum_,
the invariably great development of the adductor plate,--the marked
manner in which its basal margin is reflexed,--the absence of a
tooth-like articular projection,--and again in the tergum of _P.
crenatum_, the invariably large size of the concave spur, without
any internal tooth, altogether convince me that the two species must
be considered as distinct. This species is allied to _P. grande_, in
the scutum of that species having an occludent ledge, though small,
and a great adductor plate. I have only further to remark, that the
figure of the opercular valves, given in Sowerby's Genera of Recent
and Fossil Shells, is so good, that there can not be the least doubt
about the present identification.

9. PYRGOMA MONTICULARIÆ. Pl. 13, fig. 5 _a_-5 _f_.

PYRGOMA (DARACIA) MONTICULARIÆ. _J. E. Gray_ (!). Zoological
Miscellany, p. 6, 1831.

_Shell of an irregular shape, with a roughened exterior border: orifice minute, circular: scutum and tergum both much elongated, calcified together without any suture, both furnished with a broad occludent ledge._

_Hab._--Singapore; Mus. Brit., Cuming, and Stutchbury. Sometimes
associated with _Creusia spinulosa_.

_Appearance and Structure of Shell._--Shell dull white, very
irregular in outline, sometimes rounded, more often unequally
elongated, and frequently star-shaped,--the projections being
quite irregular. Whole shell nearly flat, but with the central
part saddle-backed, or formed into a more or less prominent ridge,
extending in the line of the longitudinal axis of the animal's
body: the circumferential portions of the shell not unfrequently
are a little recurved upwards. Orifice extremely minute, circular.
The outer lamina of shell, which is smooth, does not extend to the
circumference, and consequently a rather broad, nearly equal border,
which is rough, surrounds the whole shell. I have no doubt that,
when the shell was alive, this border was covered by a membrane,
which, in drying, has curled up and been lost, in the same manner
as the strictly analogous but narrow open seam between the basal
edges of the shell and the basis in some cirripedes (as in the last
two species of _Pyrgoma_) is protected. The roughened border can
sometimes be plainly seen to be formed of normal (Pl. 13, fig. 5
_e_) longitudinal septa having crenated edges, with shorter septa
between the longer ones; but more often the septa are so irregular,
and so much branched (5 _d_), that the whole resembles a mass of
moss. Why the outer lamina of the shell in this one species does not
nearly reach the circumference of the walls, I cannot conjecture. The
extremely irregular, depressed shape of the shell, with the minute
circular orifice, and the singular rough circumferential and often
slightly reflexed border, together give to this species so peculiar
an aspect, that until close examination I did not believe that it
was a cirripede. The extreme irregularity of shape depends in great
part upon the irregular growth of the Monticularia, in which it is
imbedded.

Internally (fig. 5 _b_) the walls are smooth, but they are perforated
by many quite irregular, small orifices, which have the appearance
of having been formed by some boring animal, but really serve, as
I believe, to admit threads of corium into certain irregular pores
which penetrate the shell. The sheath descends but a very short
distance from the orifice: it is closely attached to the walls,
and might easily be overlooked. The basis is deep, of an irregular
outline, like that of the shell, and is formed by a very thin shelly
layer. The largest specimen which I have seen, measuring from the
extreme projecting points, was .4 of an inch in diameter.

The _Scuta_ and _Terga_ (fig. 5 _f_) are calcified together without
any trace of a suture; together they form a bow with the two ends
curled rather abruptly inwards; they are both extremely narrow, but
furnished with an occludent ledge, twice or thrice as high as the
proper valves themselves. This occludent ledge, which is finely
hirsute, begins at about one third of the length of the scutum
from the rostral angle, and runs to near the basi-carinal angle of
the tergum. The scutum itself is curved, with a slip, along the
true occludent margin (best seen at the rostral end), lying in a
different plane from the rest of the valve, much in the same way as
in the scutum of _P. milleporæ_. The adductor ridge descends a very
little below the basal margin of the valve, and extends for nearly
its entire length: this adductor ridge makes the proper valve even
narrower than it at first appears. The _Tergum_ is extremely narrow,
forming merely a border to the occludent ledge; but it is not short,
being about two thirds of the length of the scutum. There is no trace
of a spur; indeed, the valve is rather narrower where the spur should
have stood, than it is at the basi-carinal end. The scuta and terga
are calcified together by their apices.

_Affinities._--Although this species, as above stated, differs so
remarkably in external appearance from the other species of the
genus, and, indeed, of the whole family, yet the shell in no one
essential point of structure materially differs from its congeners;
and if we compare the opercular valves with those of the three
last species, we shall be struck with their close, yet graduated,
affinity: in _P. milleporæ_ the scuta and terga tend to become
calcified together, and the rostro-occludent end of the scutum is
bent into a different plane from the rest of the valve. In _P.
dentatum_ we have an occludent ledge exactly as in the present
species; but in that species the adductor plate is less developed
than in _P. monticulariæ_; on the other hand, in _P. crenatum_, the
adductor plate is more developed than in _P. monticulariæ_. If in
_P. crenatum_ we were to remove the spur from the tergum (and it is
much less developed in _P. dentatum_; and in _P. milleporæ_ it is
entirely absent) this valve would be almost identical with that of
_P. monticulariæ_. Under these circumstances I consider it impossible
to separate the present species as a distinct genus.

_Species Dubiæ._

The _Daracia Linnæi_ of J. E. Gray (Annals of Philosophy, new series, vol. 10, 1825), was published without description or figure.

The _Megatrema semicostata_ of G. B. Sowerby, junr. (Conch. Manual, fig. 33, 1839), is not described, and is very indifferently figured without the opercular valves, and therefore can never be recognised.

There is an admirable figure of a Pyrgoma, without any specific name, in the great 'Description d'Egypte,' but from the want of some details, I cannot positively recognise the species; I am inclined to believe that it is the _P. dentatum_ of this work.

M. Chenu, in his grand Illustrations Conchyliologiques, has given most beautiful figures of several species of Pyrgoma, and of Creusia, but unfortunately, from the opercular valves not having been figured, I find it impossible to recognise them. The new species are _Pyrgoma stellata_ (on which I have appended a note under _P. conjugatum_), and _P. spongiarum_ and _P. corymbosa_ of Valenciennes.

6. _Sub-Genus_--CREUSIA. Pl. 13, 14.

CREUSIA. _Leach._ Journal de Physique, tom. 85, July, 1817.

_Compartments four, furnished with radii; basis cup-formed: attached to corals._

_Distribution_, imbedded in corals throughout the tropical seas.

Creusia is closely allied to Pyrgoma; and had not this genus already been adopted by several authors, I should not, I think, myself have formed it; though, no doubt, it harmonises well with some of the other genera of the family, which are perhaps all too intimately related. Creusia differs from Pyrgoma only in the shell being separated by sutures, into four compartments, with well developed radii: in other respects, such as in general habit, in the cup-formed imbedded basis (not permeated by pores), in the opercular valves, in the characters derived from the mouth and cirri, there are no generic differences. This affinity is more particularly evident when Creusia is compared with the first few species of Pyrgoma: indeed, for a short time, I was inclined to consider _var._ 10 of Creusia as identical with _Pyrgoma conjugatum_. With respect to the species of Creusia, I confess I have been much perplexed in determining whether there be only one, or half a dozen. The latter conclusion would almost certainly be arrived at if only a few specimens were examined; and it might naturally be thought that some of the species were extremely well marked; the difficulty of drawing any line between varieties and species, begins only when some hundreds of specimens, from various parts of the world, are disarticulated, cleaned, and carefully examined. Creusia, in this respect, offers a striking contrast with Pyrgoma, in which nearly all the species are strongly characterised. The shell differs very little in the several varieties or species of Creusia; and the most marked difference, namely, whether the walls are permeated by irregular pores or not, seems certainly quite variable. It is in the opercular valves, which in other genera offer by far the most reliable character, that we encounter the chief cause of perplexity; for the characters thus derived, though at first appearing very distinct, blend into each other, and are not accompanied by any well marked differences in the shell. Only a few of my specimens have any habitat; but the geographical range, as far as it does go, throws no light on the question which forms to regard as species and which as varieties. As is generally the case with cirripedes, the variations are local, so that the greater number of specimens imbedded in the same coral resemble each other. Under these circumstances I have thought it best, after repeated examinations of a very large suite of specimens, to describe separately each variety, without attaching any name to it; but I will first make a few general remarks on the structure of the shell. If I do not thus throw much light on the subject, I shall at least not burden it with error. I believe that the species will be definitely made out only by persons resident in the coral-bearing zones. I have given copious illustrations of the opercular valves; for, if my view be correct, this genus offers a curious and striking case of variation; if, on the other hand, I am wrong, the drawings, I hope, will aid others in coming to a more correct conclusion.

1. CREUSIA SPINULOSA. Pl. 13, fig. 6 _a_-6 _h_: Pl. 14, 6 _i_-6 _u_, 6 U.

CREUSIA SPINULOSA. _Leach_ (!). Encyclop. Brit. Suppl., vol. 3, Pl.
57, 1824.

CREUSIA SPINULEUSE. _De Blainville._ Dict. Sc. Nat., Pl. 116, fig. 6.

CREUSIA GREGARIA. _G. B. Sowerby_ (!). Genera of Recent and Fossil
Shells, No. 18, Sept. 1823.

---- GRANDIS. _Chenu._ Illust. Conch. Tab. 1, fig. 2, sed non fig. 2
_a_ and _b_.

_Hab._--Philippine Archipelago, China, Singapore, Java, Red Sea, West
Indies; imbedded in various corals; Mus. Brit., Cuming, Stutchbury,
Dunker, &c.

_General Appearance._--The shell is oval, generally flat, sometimes
conical, with narrow and approximate ridges radiating from the
orifice (fig. 6 _a_). The ridges, however, are sometimes distant from
each other, and considerably prominent, projecting round the basal
border. The orifice is either neatly diamond-shaped or oval. The four
compartments are quite distinct; the radii are generally white, of
considerable width, and with their summits not oblique. The colour
is either white, or pale pinkish-purple; but in _var._ 11, bright
pink. Even in the white specimens, when well preserved, the sheath is
generally, but not always, either pale or dark purple. The largest
specimen which I have seen, from the West Indies, was above half an
inch in diameter; but from .3 to .4 of an inch is the more usual full
size. I believe that the size, as well as the great variability of
the present species, is partly determined by the rate of growth of
the various zoophytes in which the specimens are imbedded, for the
shell has to keep on nearly a level with the surface of the coral.

_Structure of Shell and Basis._--The walls are internally ribbed;
the ribs being usually prominent, sometimes to such a degree as
to deserve to be called plates. The outer lamina is of variable
thickness, and the prominence of the internal ribs appears in
considerable part to depend on the extent to which the outer lamina
has been thickened from within. In many specimens, instead of the
interspaces between these internal ribs or longitudinal septa
being solidly filled up, separate and successive laminæ have been
deposited, by which the shell is rendered cancellated or porose; but
the pores are very irregular; and sometimes they form two or three
irregular rows one behind the other: this structure seems eminently
variable. The edges of the radii are formed by crenated, and
occasionally branched, septa. That part of the alæ, which is added
during the diametric growth of the shell, is very thin. The lower
edge of the sheath seems always to be free. The shelly layer, forming
the basis, which is deeply cup-formed, is thin, more or less finely
furrowed in radiating lines, and not permeated by pores.

The opercular valves will be best described under the following
eleven varieties.

_Var._ (1), Pl. 13, fig. 6 _a_, 6 _b_, 6 _c_, 6 _d_.--_Hab._ Java,
and probably several other districts.--I will first describe a
typical sub-variety, by which I mean a sub-variety not presenting
any extreme character. The scutum is of a sub-triangular shape, with
the basi-tergal corner much rounded off (6 _d_), and generally but
not always hollowed out in a rather remarkable manner. The adductor
ridge is considerably prominent, and extends high up, parallel to
the articular ridge, which latter is rounded and prominent, but
to a variable degree. Near the rostral angle there is sometimes a
small tooth, or only a trace of one, depending beneath the basal
margin; this tooth we shall hereafter see much more developed. The
_Tergum_ is about two thirds of the width of the scutum. It is
often slightly beaked, but this is more conspicuous in some of the
following varieties. The spur is about half the width of the valve,
and its basal end is truncated, and nearly parallel to the basal
margin of the valve, but the truncated form passes insensibly into
a rounded outline. The shell in this variety is generally thick and
is not permeated by pores; the orifice is diamond-shaped. But in
another sub-variety the walls of the shell are always, or nearly
always, permeated by pores, and the tergum is very much narrower,
with the spur sharper, so that at first I concluded that these two
sub-varieties were specifically distinct: we shall, however, soon
see in _var._ 2 and in _var._ 4, that no confidence whatever can be
placed in the exact breadth of the tergum, or in the porosity of the
walls; hence I have been driven to consider the two varieties just
mentioned as merely sub-varieties.

_Var._ (2), fig. 6 _e_, 6 _f_, 6 _g_.--_Hab._ China; Red Sea.--The
shell is almost invariably permeated by pores, sometimes arranged
in two or three very irregular rows. In some specimens the scutum
exactly resembles that in _var._ 1, but with the tooth near the
rostral angle often rather larger: in other specimens the scutum is
much more elongated transversely (fig. 6 _e_), with the adductor
ridge more medial, and the basal margin not at all hollowed out
at the basi-tergal corner of the valve. The tergum, here, is the
remarkable feature, being sometimes excessively narrow, with a long
sharp spur, which often, but not always, terminates in a needle-like
point. In other specimens, from the same coral and certainly
belonging to this same variety, the valve is not so narrow (6 _g_),
and the spur not so pointed; consequently (as in several analogous
cases in other cirripedes) it is impossible to draw any line of
distinction between the specimens with the narrow and broad terga.

_Var._ (3), fig. 6 _h_ [_Creusia gregaria of G. B. Sowerby!_]--_Hab._
Unknown.--The scutum presents here exactly the same considerable
range of variation as in _var._ 2. The tergum is broad, as in _var._
1, but the spur is rounded, and from not being placed so immediately
close to the basi-scutal angle of the valve, gives to it a rather
different aspect. The breadth of the spur varies; an extreme variety
is given in fig. 6 _h_.

_Var._ (4), Pl. 14, fig. 6 _i_, 6 _k_, 6 _l_.--_Hab._ Philippine
Archipelago; West Indies.--The _scutum_ here presents the same
sub-varieties as heretofore, excepting that I have not seen any so
much elongated transversely. The shell is covered either with slight,
closely approximate ribs, as in the foregoing varieties, or with more
distant and more prominent ribs. In specimens taken out of the same
branch of coral the walls were either porose or solid. Sometimes the
sheath is bright purplish-pink. It is the _tergum_, again, which
presents a remarkable range of difference; for the longitudinal
depression or furrow which in the former varieties was quite open,
here has its edges more or less folded inwards, and is sometimes
quite closed. This same variation has been commonly observed in many
species of Balanus, in which it appears to be dependent on the age
of the individual; but this does not appear to be the case in the
present genus. As a consequence of the greater or less folding in of
the two sides of the furrow, the spur is rendered more or less narrow
and pointed, and thus becomes removed to a greater or less distance
from the basi-scutal angle of the valve. Further, as a consequence of
this folding in, the internal surface of the valve along the line of
the external furrow, is raised into a longitudinal ridge. The length
of the spur varies considerably. In some very young individuals, the
basal margin descends lower on the scutal than on the carinal side
of the spur. In one set of specimens (fig. 6 _l_), a plate extended
from the carinal margin to near the central longitudinal ridge
just mentioned: a similar structure was described under _Pyrgoma
cancellatum_.

_Var._ (5), Pl. 14, fig. 6 _m_.--_Hab._ Unknown.--We have seen in
_vars._ 2 and 3 that the scutum varies considerably in shape: here
it is unusually narrow, with the adductor ridge almost touching the
articular ridge. There is no little tooth near the rostral angle, and
the basi-scutal corner is not hollowed out. The tergum also varies;
in some individuals it is truncated and like that figured of _var._
1, but rather more rounded; in other specimens (from the same branch
of coral) the basal margin so blends into the spur that the latter
can hardly be discriminated (fig. 6 _m_); in other respects the
outline resembles pretty closely that of one of the _sub-vars._ (fig.
6 _f_) of _var._ 2. The shell is not porose; it is thick, with strong
internal ribs, and resembles that of _var._ 1; but it is of a pale
purplish colour.

_Var._ (6), _an. spec._? fig. 6 _n_-6 _q_.--_Hab._ Philippine
Archipelago.--This is a very remarkable variety; we have, imbedded
in the same coral, and with shells absolutely identical, specimens
with the scutum having three distinct but graduated forms. Firstly,
a scutum transversely elongated, in all external respects like some
of the varieties mentioned under _vars._ 2 and 3, with no rostral
tooth, and not hollowed out at the basi-tergal corner, but with the
adductor ridge more prominent. Secondly, a scutum of the same general
shape, but with the adductor ridge so much developed (fig. 6 _n_) as
to descend slightly beneath the basal margin, and to be seen when
the valve is viewed externally; there is a very slight tooth near
the rostral angle (as in some former sub-varieties), and which can
be here rather more clearly seen than hitherto, to be formed by the
adductor ridge (closely united to the external surface of the valve)
extending thus far, and being here produced a little downwards.
Thirdly (6 _p_), we have the adductor ridge immensely developed,
descending far below the basal margin of the ordinary valve; and
the basal margin at the basi-tergal corner is angularly and deeply
hollowed out. The appearance of the valve is widely different from
that in the first sub-variety, yet it is impossible to separate the
first and second sub-varieties, and it is almost equally certain that
the third sub-variety is only an exaggeration of the second. The
lower edge of the adductor ridge, in the third sub-variety, varies
a little in outline; it is deeply sinuous, and is produced at the
rostral angle into a point, of which we have heretofore seen only a
feeble representation. It would appear as if the great development of
the adductor plate had caused the exterior ordinary surface of the
valve to shrink or be less developed. There is a striking resemblance
in the structure here described with that in _Pyrgoma cancellatum_
and _conjugatum_. The terga belonging to the above scuta, also,
vary; the spur being sometimes square (6 _o_), and sometimes bluntly
pointed: when the spur runs in the same exact line (6 _q_) with the
scutal margin of the valve, a peculiar aspect is given to it, but
this is by no means always the case. Both opercular valves are often
partially coloured pinkish-purple. The shell is not porose; it is
thin, with remarkably prominent internal plates; it is apparently
always of small size, which I attribute to this variety inhabiting
a hard thin plate-like coral. The sheath is bright pinkish-purple,
of which we have had instances in some of the other varieties; and
the shell itself is sometimes pinkish. Taking the scutum of the
first sub-variety, together with the commonest accompanying variety
of tergum, I find it quite impossible to assign to it a specific
character; if, on the other hand, we consider the scutum of the
third sub-variety by itself, nothing can appear more distinct; but
I must repeat, there can be hardly a shadow of doubt that the three
sub-varieties of scutum here described, graduate into each other, and
are specifically identical.

_Var._ (7), fig. 6 _r_.--_Hab._ Probably Philippine Archipelago,
associated with _Balanus quadrivittatus_.--There can be hardly any
question of this being specifically identical with the last variety.
It inhabits a different coral. All the specimens were of small size.
The walls are not so thin, and the internal ribs not so prominent as
in _var._ 6. The sheath is either white or dull purple; I can, in
short, point out no difference in the shell from the typical _var._
1. The scutum is not so much elongated transversely as in _var._
6, and the basi-tergal corner is more cut off,--in which respect
it resembles the common varieties. The adductor ridge is largely
developed, so as to be just visible when the valve is viewed from the
outside, in a degree between the first and second sub-varieties of
_var._ 6: but the most singular character is the larger development
of the tooth near the rostral angle, and this was the case in the
same degree in all the specimens which I examined. The tergum
resembles that rather unusual sub-variety of _var._ 6 (fig. 6 _q_),
which has the scutal margin and the one side of the spur forming a
straight line. It appears to me that it would be absurd to consider
these slight differences, in parts unquestionably subject to much
variation, as specific, when we are almost forced to admit that the
much greater differences in the three sub-varieties of _var._ 6, are
not of specific value.

_Var._ (8).--_Hab._ Unknown, Mus. Cuming.--I have seen only a single
specimen of this, and refer to it on account of _var._ 11. The shell
is rather steeply conical, with distant and prominent ribs; the radii
are narrow; the walls are not permeated by pores; the colour is pale
purple. Altogether its external appearance is very different from
that of the foregoing varieties; but the scuta are identical with
those of _var._ 1, excepting that the rostral tooth is rather larger,
being nearly as large as in the last, _var._ 7. The tergum precisely
resembles that in some specimens of _var._ 2. Hence this variety
differs from the first two varieties only in the shade of colour, the
external shape, and the greater prominence of the external radiating
ribs of its shell. All these characters are variable in the several
foregoing varieties, and they have been found, as yet, insufficient
to discriminate species in any genus of sessile Cirripedes.

_Varieties With the Scuta and Terga Calcified Together._

_Var._ (9), Pl. 14, fig. 6 _s_ [_C. spinulosa_, of Leach (!)]--_Hab._
Unknown.--The shell is undistinguishable by a single character from
many specimens of the first, third, and fourth varieties; it is not
permeated by pores. The scutum and tergum, with the exception of the
one striking difference of their being calcified together without
any trace of a suture, are identical with those of _var._ 3, as may
be seen by comparing the figures 6 _h_ and 6 _s_. Hence to separate
this form specifically from _var._ 3, we should have to rely solely
on the calcification or union of the scuta and terga; but we have
seen this is a point which is variable in _Elminius Kingii_, _Pyrgoma
milleporæ_, and in some species of Balanus. The serial affinities,
moreover, in Pyrgoma, clearly show that this is a character of no
great importance. I must add that in several specimens of several of
the varieties, the scuta and terga were so closely joined, that until
careful examination, I was unable to detect the suture separating
them; such being the case it must be quite unimportant for any
functional purpose, whether or not the valves are calcified together.
I feel, consequently, hardly any doubt that I have acted right in
treating the present form as a mere variety.

_Var._ (10) _an. spec._? fig. 6 _t_.--_Hab._ Unknown.--This variety
bears nearly the same relation to _var._ 6, as the last variety did
to _var._ 3. The shell is rather stronger than in _var._ 6, with the
internal ribs not so prominent; and except in being tinted pale dull
purple, it differs in no respect from the shell of _var._ 1. If we
imagine the scutum and tergum in the third sub-variety of _var._ 6
(6 _p_, 6 _q_), in which the adductor ridge descends far beneath the
true basal margin of the valve, to be calcified together, without any
suture, we shall produce almost the identical valves of the present
variety. The scutum, however, here is not quite so much elongated
transversely, and the occludent margin is spinose and is furnished
with large teeth; these two characters give the valve a somewhat
different aspect, and hence I am more doubtful than in the foregoing
case, whether this form may not be specifically distinct. I must,
however, state that in _Tetraclita porosa_, I ascertained that the
teeth on the occludent margin of the scuta were even more variable
than here is supposed to be the case; and as for the shape of the
valve we have seen what wonderful variation there is in _var._ 6. The
tergum in this variety is about intermediate between the two common
forms, in the sub-varieties of _var._ 6. As for the calcification of
the two valves together, we have seen, under the last variety, how
little important a character it is.

_Var._ (11) fig. 6 U, 6 _u_ [_Creusia grandis_, of Chenu, Tab. 1,
fig. 2, but not fig. 2 _a_ and _b_].--_Hab._ Singapore, associated
with _Pyrgoma monticulariæ_.--This variety is very closely related
to the last. The shell, however, has a very peculiar aspect, which
made me for some time think it must be specifically distinct. It is
of a much brighter pink than in any of the foregoing varieties; the
surface is marked with very prominent, distant ribs, and the radii
are narrow, in which latter points, together with the tint (though
here brighter), this variety cannot be distinguished from _var._ 7.
The shell, however, is permeated by several rows of pores, in which
respect it resembles the shell in _var._ 2, and some specimens of
_vars._ 3 and 4. In the opercular valves there is a close general
resemblance with those of the last _var._ 10; the tooth, however,
near the rostral angle, is not so prominent; and in the tergum, the
spur is more truncated, shorter and broader than in _var._ 10, and
closely resembles that in _var._ 1 and 2. But I cannot consider any
of the points here specified of much weight.

The foregoing descriptions show how singularly the affinities of the
several varieties interlock in the most complicated manner. Hereafter
some one may, perhaps, succeed in grouping several of these forms as
species; but I am sure he ought not to attempt it without possessing
a very large suite of specimens, or without the great advantage of
comparing some two or three of the forms, fresh in their native site.

_Species dubiæ._--Under Pyrgoma, I have stated that though Chenu,
in his 'Illust. Conch.,' has given beautiful external figures of
the shells, imbedded in the coral, yet from the want of details on
the opercular valves and on the structure of the shell, I cannot
recognise his species. So it, likewise, is with Creusia. Chenu gives
the following new species; _C. radiata_, _multistriata_, _decorata_,
and _striata_. The _C. madreporarum_, I suspect to be the _Pyrgoma
milleporæ_ of this work, as there stated. The _C. grandis_ no doubt
is the _P. grande_ of this work, the _Nobia grandis_ of Sowerby. The
name _Creusia Childreni_ is given by Dr. Gray, without description or
figure, in the 'Annals of Philosophy,' vol. 10, new series, 1825.

7. _Genus_--CHELONOBIA. Pl. 14: Pl. 15, fig. 1.

CHELONOBIA. _Leach._ Journal de Physique, tom. 85, (1817).

CORONULA. _Lamarck._ Animaux sans Vertèbres, 1818.

---- _Ranzani._ Memoire di Storia Naturale, 1820.

---- _De Blainville._ Dict. des Sciences Naturelles.

ASTROLEPAS. _J. E. Gray._ (Klein) Annals of Philosophy, (new series),
vol. 10, (1825).

_Compartments extremely thick, six; but one of them, the rostrum, internally is composed of three rudimentary compartments, united together: basis membranous: scuta narrow, united to the terga by a horny articular ridge._

_Distribution_, throughout the tropical, and warmer temperate seas of
the whole world; attached to turtles, crustacea, or smooth gasteropod
molluscs.