191 



and characteristic distinction of its being heterostro- 

 phe, or what is commonly called a reverse shell, which 

 also occurs in many other examples examined by the 

 writer. 



Planorbis cornu-arietis 



corneus 



carinatus 



lutcscens 



orientalis 



• • spirorbis 



Planorbis vortex 



deformis 



contortus 



hispidus 



nitidus 



inibricatus 



PHYSA. 



BULLA FONTINALIS. Linn. 



: 3 



Sowerhys Genera, No. 1 >j)late§. 

 The G. Physa, established by Draparnaud, is a fresh 

 water shell, thin, fragile, and generally heterostrophe, 

 or having the whorls turned to the left hand; they were 

 considered Bullae by Linnaeus, from which the project- 

 ing spire distinguishes them: tiiey are nearly allied to 

 the G. Lymnaea, from which they only differ in not 

 having the aperture widened, the right margin being 

 a little advanced above its base. Sowerby in his Ge- 

 nera of Shells, No. 8, has united this genus with that 



