_C. Danmoniensis._ The Devonshire Crassina. Pl. 6, fig. 1. 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. 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.