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.