Solid, thick, sub-orbicular, sub-equilateral; two very large divergent teeth on one valve, and two very unequal ones on the other; regular parallel grooves and ribs; covered with a yellowish epidermis; inside white; margin broad and plain.
FAMILY IX.
Conchacea. Seven genera.
This family is divided into Conchæ fluviatiles, fresh-water shells, and Conchæ marinæ, sea shells.
C. Fluviatiles. Three genera.
1. Cyclas. Eleven species.
The shells of this genus are very small, and are found buried in the mud of fresh waters; the apices or summits are never eroded, and some species are so thin as to be transparent.
Shell covered with a brown epidermis, oval or sub-orbicular, regular, equivalve, inequilateral; summits blunt, contiguous, or turned anteriorly; hinge similar, complex, formed by a variable number of cardinal teeth, and by two remote lateral teeth with a cavity at the base; ligament exterior, posterior, and convex; two distant muscular impressions, without posterior sinus.
Cyclas rivicola.
C. cornea.
C. lacustris.