CATOPHRAGMUS POLYMKRUS. 487 



the coriura is prolonged for some way upwards, are of 

 service to the animal, by thickening its shell, in an analo- 

 gous, but not homologous, manner, as in Tetraclita. 



Considering the whole structure, external and internal, 

 of Catophragmus, with the one great exception of the ex- 

 terior 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. polymeries, 

 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 oper- 

 cular 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<z — 4 e. 

 Basis membranous : caudal appendages none. 



Hob. — New South Wales (Twofold Bay), Mus. Darwin; Swan River (?), 

 Mus. Cuming. Attached to littoral rocks and shells, and associated with 

 Tetraclita purpurascem, Balanus nigrescens, Chthamalus antennatus, Chamasipho 

 columna. 



General Appearance. — Shell nearly circular, moderately or slightly 

 depressed ; colour grey. The eight normal compartments of the 

 inner whorl are two or three times as large as those in the second 

 whorl ; the other smaller compartments or scales graduate very regu- 



