Mandibles, with three main teeth, with either one or two smaller teeth inserted between the first and second, making four or five altogether; inferior angle rather narrow, pectinated with long and fine spines.

Maxillæ, rather broad, with two long upper spines; beneath which there is a very small prominence bearing a minute tuft of fine bristles; beneath this, there are eleven pairs of rather long and strong spines; and the inferior angle is formed by a rather broad, upraised, and obliquely rounded prominence, bearing a broad tuft of fine spines.

Outer Maxillæ, with the inner surface continuously clothed with short spines; exteriorly there is a slight prominence with long hirsute spines.

Olfactory Orifices barely prominent.

Cirri.—First pair placed near the second; the segments of the three posterior pairs are slightly protuberant, and bear three or four pairs of finely serrated spines; intermediate tufts long, the middle spines being the longest; spines on the upper lateral edges long and strong; dorsal tufts rather short. First cirrus, long, multiarticulate, having fourteen or fifteen segments, whilst the sixth cirrus had nineteen segments; rami unequal in length by about two segments; basal segments protuberant brush-like. Second and third cirri with five basal segments of both rami protuberant and brush-like; but the anterior rami in both cirri are broader than the posterior rami. Spines on the protuberant segments of both rami of both cirri, coarsely and doubly pectinated.

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.]