Cyclostoma planorbula. C. volvulus. C. carinata. C. sulcata. C. unicarinata. C. tricarinata. C. obsoleta. C. rugosa. C. labeo. C. interrupta. C. ambigua. C. semilabris. C. flavula. C. patulum. C. fasciata. C. mumia. C. quaternata. C. ferruginea. C. decussata. C. lineolata. C. mamillaris. C. ligata. C. lincinella. C. orbella. C. fimbriata. C. multilabris. C. elegans. C. truncatulum. _C. elegans._ The elegant Cyclostoma. Pl. 19, fig. 5. Species with spire slightly elevated, ovate, conical, umbilicated; volutions convex; finely striated transversely. _C. fasciata._ The banded Cyclostoma. Species with spire very elevated. _C. Planorbula._ The Planorbis-shaped Cyclostoma. Species with spire very depressed. FAMILY VIII. LYMNÆCEA. Three genera. 1. Planorbis. Twelve species. Taken from the Helix of Linnæus to distinguish the aquatic from the terrestrial shells. This genus is found in fresh water, and has no operculum. Shell thin, often sinistral, discoid, or involuted almost in the same vertical plane; the spire not projecting and entirely lateral, so that the shell is hollowed or depressed on each side; aperture small, transverse, with edges sharp, not reflected, disunited by the last whorl of the spire which modifies it; sometimes carinated. Planorbis cornu-arietis. P. corneus. P. carinatus. P. lutescens. P. orientalis. P. spirorbis. P. vortex. P. deformis. P. contortus. P. hispidus. P. nitidus. P. imbricatus. _P. carinatus._ The keeled Planorbis. Species with a keel; depressed, upper side concave. _P. corneus._ The horny Planorbis. Pl. 20, fig. 4. Species without a keel. 2. Physa. Four species. This genus is generally heterostrophe (that is, with whorls turned to the left hand); found in fresh water; it greatly resembles the Lymnæa, but has not a widened aperture.