CATOPHRAGMUS. _G. B. Sowerby._ Genera of Recent and Fossil Shells.
Plate.
_Interior compartments eight, with several exterior whorls of small
supplemental compartments: basis either membranous or calcareous._
_Distribution_, West Indies and Australia. Attached to littoral
shells and rocks.
This genus is very remarkable amongst sessile cirripedes, from the
eight normal compartments of the shell being surrounded by several
whorls of supplemental compartments or scales: these are arranged
symmetrically, and decrease in size but increase in number towards the
circumference and basal margin. A well preserved specimen has a very
elegant appearance, like certain compound flowers, which when half open
are surrounded by imbricated and graduated scales. The Chthamalinæ, in
the structure of the mouth and cirri, and to a certain extent in that
of the shell, fill up the interval between the Balaninæ and Lepadidæ;
and Catophragmus forms, in a very remarkable manner, the transitional
link, for it is impossible not to be struck with the resemblance of
its shell with the capitulum of Pollicipes. In Pollicipes, at least
in certain species, the scuta and terga are articulated together--the
carina, rostrum, and three pairs of latera, making altogether eight
inner valves, are considerably larger than those in the outer
whorls--the arrangement of the latter, their manner of growth and
union,--all are as in Catophragmus. If we, in imagination, unite some
of the characters found in the different species of Pollicipes, and
then make the peduncle so short (and it sometimes is very short in _P.
mitella_) that the valves of the capitulum should touch the surface of
attachment, it would be impossible to point out a single _external_
character by which the two genera in these two distinct families could
be distinguished: but the more important differences in the arrangement
and nature of the muscles which are attached either to the opercular
valves or surround the inside of the peduncle, would yet remain.
Although all the valves of the shell, even the eight in the innermost
whorl, are very thin, yet from their number in the successive whorls,
and from each being concave inwards, so as to form a cavity or tube
into which the corium enters, the total thickness of the sides of
the shell is very considerable. Both of the species of Catophragmus
occurred mingled, in the one case with _Tetraclita porosa_ and in the
other with _T. purpurascens_; now the walls of these shells, we know,
are very thick, and are permeated by several rows of pores, occupied by
threads of corium; seeing this, we may be permitted to believe, that
the several exterior whorls of valves in Catophragmus, between which
the corium is prolonged for some way upwards, are of service to the
animal, by thickening its shell, in an analogous, but not homologous,
manner, as in Tetraclita.
Considering the whole structure, external and internal, of
Catophragmus, with the one great exception of the exterior whorls of
valves, there is hardly a single generic character by which it can be
separated from Octomeris and Pachylasma; indeed, I am not quite sure
that it would not have been better to have run these three genera
together.
Of the two species, I will first describe _C. polymerus_, and
not the _C. imbricatus_ of Sowerby, inasmuch as I have plenty of
excellent specimens of the former, whereas the original specimens of
_C. imbricatus_, in the British Museum, consist of one old and not
perfect shell, without the opercular valves or the included animal's
body; and the other, though quite perfect, far from mature. As far as
these materials allow of minute comparison, the whole shell, with the
exception of the basis, and the opercular valves agree very closely in
the two species, whereas the included animal's body differs more than
is usual in nearly related species;--thus, _C. imbricatus_ has caudal
appendages, of which there is no trace in _C. polymerus_, and I have
seen only one other instance in which this organ was absent in one
species (_Scalpellum villosum_) and present in the other species of the
same genus. Under these circumstances it will be most convenient first
to describe in detail _C. polymerus_, and then only indicate the points
of difference in _C. imbricatus_.