94 



PROCEEDINGS OP THE ACADEMY OF 



[1881. 



trustworthy correspondent near London. This latter is the only 

 large European species, I am familiar with. 



While it is neither proven nor asserted herein, that all species 

 of Planorbis liave sinistral shells, neither is it known that the 

 anatomy of all the species is sinistral. 



The relations of Planorbis to Limnsea are not so remote as to 

 make it altogether unwarrantable to look for a divergence in that 

 direction. 



The extreme variableness of Planorbis has undoubtedly led to 

 the making of too many species ; specimens which are conceded 



Fig. 1 



Fig. 3(7. 



P. trivolvis. Say. 



P. suhcrenatus, Cpr. 



Fig. 4. 



Fig. 4a. 



P. ammon, Gld. 



P. subcrenatus, Cpr. 



P. corpulentus, Say. 



to be of the same species, from difierent though adjacent ponds, 

 etc., vary more or less, and this is particularly the case with "West 

 American forms which are in various degrees affected by the 

 character of the water, temperature, etc. While it is quite certain 

 that the specific names herein given would be reduced b}' a careful 

 and philosophical comparison, at this time I can onl^^ refer to 

 them as they now stand in the books ; I may mention P. tumens 

 from near Petaluma as a dwarfed variety of P. corpulentus ; varie- 

 ties of the latter are frequently confused with P. ammon. 



