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.