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.