68 GENERA OF SHELLS. 



LYMNEANA. 



Shell spirivalve, the external surface mostly smooth ; the right 

 margin of the aperture always sharp and not reflected. 



Planorbis. Shell discoidal, spi»e depress- 

 ed, scarcely prominent ; whorls apparent above 

 and below ; aperture oblong, lunated, very dis- 

 tant from the axis ; margin never reflexed. 

 Operculum none. 



Fresh water sii*^lls, generally thin, brittle, diapha- 

 nous ; the whorls of bome are siibcylindrical, of others 

 carinate or angular. Apevture sublongitudinal, with an 

 internal projection formed hy the penultimate whorl. 



P. cornu-arietis, corneus, carinatos, lutescens, orientalis, 

 spirorbis, vortex, deformis, contortus, i.ispidus, nitidus, im- 

 bricatus — armigerus, trivolvis, bicarinatus, parvus, paralle- 

 lus, exacuus, campanulatus, glabratus. 



Physa. Shell convolute, or oblong ; spire 

 prominent. Aperture longitudinal, narrowing 

 superiorly. Columella twisted. Lip very thin, 

 sharp, produced somewhat beyond the plane 

 of the aperture. No operculum. 



Fresh water shells, thin, brittle, and generally re- 

 versed ; distinguished from bulla by their projecting 

 spire, and from lymnœa by the aperture not being dila- 

 ted, the right margin projecting a little above its plane» 



P. castanea, fontinalis,hypnorum, subopaca, rivalis — gy- 

 rina, elongata, heterostropha, ancillaria. 



Lymnjea. Shell oblong, sometimes tur- 

 reted ; spire prominent. Aperture entire, Ion 

 gitudinal. Lip sharp, its inferior part rising 



