116 CONCHOLOGY. 



C. elegans. The elegant Cyclostoma. PI. 19, fig. 5. 

 Species with spire slightly elevated, ovate, conical, umbii- 

 icated ; volutions convex ; finely striated transversely. 



C. fasciata. The banded Cyclostoma. 

 Species with spire very elevated. 



C. Planorhula. The Planorbis-shaped Cyclostoma. 

 Species with spire very depressed. 



FAMILY VIII. 



Lymn^cea. Three genera. 



1. Planorbis. Twelve species. 

 Taken from the Helix of Linneeus 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 re- 

 flected, disunited by the last whorl of the spire which modi- 

 fies it ; sometimes carinated. 



Planorbis cornu-arietis. Planorbis vortex. 



P. corneus. P. deformis. 



P. carinatus. P. contortus. 



P. lutescens. P. hispidus. 



P. orientalis. P. nitidus. 



P. spirorbis. P. imbricatus. 



P. carinatus. The keeled Planorbis. 



Species with a keel ; depressed, upper side concave. 



P. corneus. The horny Planorbis. PI. 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 re- 

 sembles the Lymnaea, but has not a widened aperture. 



