_Caudal Appendages_ (Pl. X, fig. 19), minute, uniarticulate, club-shaped, with the enlarged ends directed inwards, or towards each other; summits sparingly clothed with very short spines. _Penis_, small. _Affinities._--This species makes a very close approach in the general form and relative sizes of all the valves, and in the variability of the number of the whorls, to _P. spinosus_; there is a still closer and more important resemblance, in the inequality and manner of growth of the calcareous scales on the peduncle. These species differ, in the colour of the membrane covering the valves, and in the greater development of both rostrum and sub-rostrum in _P. sertus_. The rostrum of the latter is longer than half the length of the carina, and its inner surface is more than twice as high as wide; and the sub-rostrum is twice as large as any of the latera,--all points of difference from P. _spinosus_. In the characters of the mandibles, and more especially of the outer maxillæ; in the length of the first pair of cirri; in both rami of the second and third cirri having their basal segments brush-like, with pectinated spines; and in the shape of the caudal appendages, there is a close relationship to _P. spinosus_, and through this species to _Scalpellum villosum_. In the little prominence of the olfactory orifices, P. _sertus_ differs from most of the allied forms, excepting _P. spinosus_. In the maxillæ having two prominences bearing fine tufts of bristles, in the roughened knobs on the prosoma, and in the presence, in some individuals, of two or three whorls of valves under the carina and rostrum, there is a marked tendency in _P. sertus_ to approach _P. cornucopia_, _P. elegans_, and _P. polymerus_. _Genus_--LITHOTRYA. Pl. VIII, IX. LITHOTRYA. _G. B. Sowerby._ Genera of Shells, April 1822. LITHOLEPAS. _De Blainville._ Dict. des Scienc. Nat., 1824. ABSIA.[65] _Leach._ Zoological Journal, vol. ii, July 1825. BRISNÆUS et CONCHOTRYA. _J. E. Gray._ Annals of Philosophy, vol. x, (new series,) August 1825. LEPAS. _Gmelin._ Systema Naturæ, 1789. ANATIPA. _Quoy_ et _Gaimard_. Voyage de l’Astrolabe, 1832. [65] The description of Absia is so inaccurate, that I should not have recognised it, had not the _Lithotrya Nicobarica_, in a bottle in the British Museum, borne this name. _Valvæ 8, si inter eas parvum (sæpe rudimentale) rostrum et duo parva latera numerentur; incrementi lineis concinnè crenatis: pedunculus squamis calcareis parvis vestitus, in verticillis superioribus crenatis; aut calyci basali calcareo aut discorum ordini affixus._ Valves 8, including a small, often rudimentary rostrum and a pair of small latera: lines of growth finely crenated. Peduncle covered with small calcareous scales, those of the upper whorls crenated; attached either to a basal calcareous cup, or to a row of discs.