14 PHYSAD.E. 



norbis, is not so large as in Physa and Limnea, 

 and has a firmer texture. I have known the 

 ova to become matured in two or three weeks, 

 but the time must vary with the species and 

 temperature. 



The different sub-generic sections in this 

 family, are, probably, nearly as indicated in the 

 following table. 



PHYSA. Shell reversed, mantle lobed, pi. 1. 

 PHYSELLA, H. With branchiae, shell globose, pi. 5. 

 APLEXA, Fleming. Lengthened, mantle simple, pi. 5. 

 PHYSODON, H. Columella toothed, pi. 5. 

 DIASTROPHA. Shell umbilicated, no fold. 



LIMNEA. Shell lengthened, pi. 5. 

 BULIMNEA, H. Shell thick in texture, inflated, pi. 3. 

 ACELLA, H. Shell slender, whirls very oblique. 

 RADIX, Montf. Shell short, greatly expanded, pi. 11. 

 AMPHIPEPLEA, Nilss. Mantle lobed. Vide LIM., p. 3. 

 ? LEPTOLIMNEA, Sw. Nearly cylindrical, spire thick. 



PLANORBIS. Whirls few and regular, pi. 1. 

 PLANORBINA, H. Whorls numerous, nearly equal. 

 PLANORBELLA, H. Aperture campanulate, pi. 1. 

 PLANORBULA, H. Aperture armed with teeth. 

 SEGMENTINA, Fleming. Cavity with open partitions. 



ANCYLUS. Apex of the shell upon the right.* 

 VELLETIA, Gray. Apex of the shell upon the left.* 



* Some authors may think it necessary to subdivide each 



