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.