86 CALIFORNIA ACADEMY OF SCIENCES. 



ing snow are much colder than the ponds, which reach 

 a heat of 50° or more in summer. 



In referring this form to P. siihcrcnatns Carpenter, we 

 have considered it as an alpine variety of the most similar 

 and first described species of its group from the west 

 coast. Taking P. coj-neus as type of the genus Planorhis 

 we find it to be a nearly perfect cylinder, coiled however 

 in a plane inclined to the left, so that its two sides are dis- 

 similar, unlike some species which have them nearly or 

 quite alike. Variations from this type in large American 

 species consist chiefly in the coarser growth lines, not 

 concealed by a thick epidermis, and in more or less lat- 

 eral narrowing or angulation of whorls. 



Carpenter in describing P. siihcrenatiis evidently com- 

 pared it with P. corneus, being little acquainted with 

 American species, and gives undue importance to the 

 coarse growth-lines which he calls "occasionally minute- 

 ly crenulated ridges," also quoting from Cuming's man- 

 uscript that it "differs from trizwivis in the acuteness of 

 the ribs, and in their being more distant." These char- 

 acters would not be considered of much specific value by 

 American authors, but they show some difference from 

 P. glabratiis, often considered a variety of trivolvis, and 

 which otherwise comes nearest to Carpenter's species. 

 It will be observed that he makes no mention of any 

 carina nor angle on the side or mouth of the shell, no 

 such character being shown in the figure by Binney, said 

 to be from the type. 



Seven years later he states that "it is quite possible 

 that this may prove a very finely grown specimen of P. 

 lentils. Dr. Kennerly's shells are intermediate." (See 

 ^'Mollusks of Western North Amer.," Smithsonian Re- 

 print, p. 675 (161), which gives his latest known opin- 

 ions, in 1872.) He does not give reasons for this belief, 



