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. C. MARINÆ. Four genera. 4. Cyprina. Two species. This shell is generally large, resembling the Venus, from which it may be distinguished by having on the front side one impressed lateral tooth, which is sometimes obsolete; the nymphæ or callosities of the hinge large, arched, and terminated near the apices by a cavity, sometimes very deep. De Blainville says that this genus is intermediary to the Cyclas and the Venus, and contains but one living species; Lamarck makes two, though at first he characterized eight. Cyprina tennistria. Cyprina Islandica. _C. Islandica._ The Icelandic Cyprina. Pl. 9, fig. 2. Shell thick, regular, heart-shaped, covered with dark brown epidermis; white interior; sub-striated longitudinally; apices very strongly recurved anteriorly, and often contiguous; hinge thick, sub-similar, formed by three cardinal teeth but little convergent, and by one remote posterior lateral tooth, sometimes obsolete; ligament very thick, convex, fixed to large, arched, nymphal callosities, preceded by a cavity more or less deep, hollowed immediately behind the summits; muscular impressions subcircular and very distant. 5. Cytherea. Seventy-eight species. This genus was taken from the Venus, and is easily defined as distinct from it by having four primary teeth on one valve, and only three united on the other, with an isolated cavity, oval and parallel to the margin; the lateral teeth divergent to the summit. In some species the internal margin is entire, having the anterior cardinal tooth with a striated canal or uneven sides; in others the anterior cardinal tooth is entire, without a striated canal; sometimes the internal margin is crenulated or dentated.