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.