This genus is found in fresh waters, and differs from the Trigonia in
the number and position of the lamellar teeth. The substance of the
shell is pearly.
Shell sub-trigonal, equivalve, inequilateral; umbones eroded, covered
with epidermis, and inflected anteriorly; hinge with two lamellar teeth,
transversely striated, one of them posterior, distant, and shortened,
the other anterior, lengthened, and lateral; ligament exterior.
_C. ambigua._ The ambiguous Castalia. Pl. 11, fig. 5.
Short, sub-trigonal; umbones truncated; longitudinally ribbed, with
distant transverse striæ; covered with epidermis, under which the shell
is of a pale chestnut brown, inside pearly; the lamellar and præ-apicial
teeth are well marked, more regular, and all striated perpendicular to
their length.
FAMILY XIII.
NAIADES. Four genera.