90 CALIFORNIA ACADEMY OF SCIENCES. 



whether they reproduce in these cases, or whether ova 

 brought on the feet of water birds merely hatch and die 

 when the drying up or stagnation of the water stops their 

 growth. There are, however, many dwarfed races or 

 subspecies of fresh water molkisca produced by unfavor- 

 able environments more or less permanent. 



As tending to prove that the species here mentioned 

 are not merely local races of one 3-whorled Planorbis, 

 it may be stated that P. suhcrenatns has been identified 

 from Honey Lake, Cal., and Nevada, by R. E. Call in 

 1884, and P. hornii was received by Tryon from Grant's 

 Lake, on the Oregon-California boundary, in 1866 (col- 

 lected by W. M. Gabb). both localities near the Sierra 

 Nevada. • 



Going back to the earliest known fossil Planorbcs, 

 we find that P. vetermis M. & H. of the Jurassic 

 of Nebraska was (like P. vcnnicularis) only 0.16 inch 

 in length, and its section shows also four whorls of the 

 simplest cylinder form. About fourteen other species 

 have been found in the Laramie, Eocene and Miocene 

 strata, all quite unlike living American forms. One of 

 late tertiary age has been found in California, which is 

 more like them, and the great extinct lakes of the Cen- 

 tral Basin contained onlv species now living near them. 



