Chapter 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.
C. obliqua.
C. calyculata.
C. obtusalis.
C. fontinalis.
C. Australis.
C. sulcata.
C. striatina.
C. Sarratogea.

_C. cornea._ The horny Cyclas. Pl. 9, fig. 7.

Species sub-orbicular, convex, thin, pellucid, with fine concentric striæ; cardinal teeth a little variable, always very small, and sometimes obsolete; summits not eroded, covered with a horn-coloured epidermis; interior bluish white.

2. Cyrena. Ten species.

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.

3. Galathea. One species.

This beautiful shell is found in fresh waters, and is distinguished from the Cyrena by the divergent form of the primary tooth.

_G. radiata._ The radiated Galathea. Pl. 6, fig. 8.

Shell equivalve, sub-trigonal, covered with a greenish epidermis, beneath which the surface is of a milky white, highly polished, with several violet or pale chestnut rays diverging from the apex to the margin; primary teeth furrowed, two on the right valve joined at the base, three on the other valve placed triangularly, the intermediate one being advanced, separate, thick, and callous; the muscular impressions are lateral, and appear double on each side.