Pinna rudis. P. flabellum. P. seminuda. P. angustina. P. nobilis. P. squamosa. P. marginata. P. muricata. P. pectinata. P. saccata. P. varicosa. P. dolabrata. P. ingens. P. vexillum. P. nigrina. _P. squamosa._ The scaly Pinna. Species very close and rounded at the posterior extremity; valves convex, covered with vaulted imbricated scales. _P. flabellum._ The fan Pinna. Pl. 13, fig. 2. Species gaping at the posterior extremity, which is as if truncated; valves rather rounded at the upper end, and in the shape of an expanded fan; light fawn colour. _P. pectinata._ The pectinated Pinna. Pl. 13, fig. 1. Thin, pellucid; longitudinally ribbed and spinous for half its width, obliquely striated transversely on the other half. FAMILY XVII. MALLEACEA. Five genera. 1. Crenatula. Seven species. There is one peculiar distinction between this genus and the Perna; the hinge of the Crenatula is composed of slightly concave callous crenulations, which receive the ligament; while in the Perna it consists of parallel truncated linear teeth (or, rather, riblike joints), corresponding and opposed to the opposite ones, the ligament being inserted only in their interstices. Shell thin, very delicate, irregular, valves flattened, foliaceous, sub-rhomboidal, sub-equivalve; hinge longitudinal, dorsal, without teeth; ligament sub-multiple, and inserted in a series of rounded cavities corresponding with the dorsal margin; one sub-central muscular impression. Crenatula avicularis. C. modiolaris. C. nigrina. C. bicostalis. C. viridis. C. mytiloides. C. phasianoptera. _C. avicularis._ The avicular Crenatula. Pl. 14, fig. 2. Answers to the above description. _C. mytiloides._ The muscle-shaped Crenatula. Oblong, ovate, oblique; base acute; violet coloured, with obscure radiations. 2. Perna. Ten species. In speaking of the Crenatula, we gave the characteristic distinction, which regarded it natural and expedient to make the Perna form a different genus. Shell irregular, very compressed, foliaceous, sub-equivalve, rather variable form, gaping at the anterior part of the lower extremity; summit very small, hinge straight, vertical, without teeth; ligament multiple, and inserted in a series of longitudinal parallel furrows.