This genus of shells is found in rapid rivers and streams; it was
formerly classed with the Cyclas, from which, however, it greatly
differs in having three cardinal teeth on each valve, and also lateral
teeth. They are thick, solid shells, sometimes of a large size; the
apices always eroded or carious.
Shell rounded and trigonal, ventricose, inequilateral; hinge with three
teeth on each valve; two lateral teeth, one of which is near the primary
ones; ligament exterior, placed on the largest side; in some species the
lateral teeth are crenulated, in others they are entire.
Cyrena trigonella.
C. orientalis.
C. cor.
C. depressa.
C. Caroliniensis.
C. fuscata.
C. fluminea.
C. violacea.
C. Bengalensis.
C. Ceylanico.
_C. fluminea._ The river Cyrena. Pl. 6, fig. 7.
Species sub-trigonal or elongated oval; summits decorticated, more
anterior; three cardinal teeth, of which the two posterior are forked;
exterior greenish brown, interior variegated with white and violet;
sulcated transversely.