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].
1. [CATOPHRAGMUS] POLYMERUS. Pl. [20], fig. [4 a]-[4 e].
Basis membranous: caudal appendages none.