Correct Position of the Aperture in Planorbis. 47 



Planorbis bicarinatus Say, is very difficult to decide 

 upon from the adult shells, for the entire number of whorls 

 may be counted on both sides. It was only by reducing the 

 shells to two whorls that I was enabled to absolutely deter- 

 mine its sinistral character. I was much astonished, also, to 

 find that these dissected shells corresponded exactly to num- 

 erous very small Planorbes which have puzzed me for some 

 time. It is more than probable that all the young of Plan- 

 orbis show distinctly their sinistral character. 



Planorbis subcrenatus Cpr., is also sinistral, but is 

 only seen to be such after dissection. 



Planorbis glabratus Say. This is very closely allied 

 to trivolvis, and, like that species, it is sinistral. 



Planorbis ammon Gould, is sinistral, a fact easily ascer- 

 tained by a close examination, even without the trouble of a 

 dissection. By breaking away to the second whorl, the sinis- 

 tral character is seen to be quite pronounced. The umbilical 

 region is much indented. 



Planorbis corneus Lime, is sinistral. 



Planorbis corpulEntus Say, is sinistral, and, if I mis- 

 take not, will prove to be a synonym, or at most a variety, of 

 trivolvis. The two species merge into one another, as may 

 be seen by examining a large series of both. 



Planorbis lentus Say. This species is sinistral, and is 

 also very closely related to trivolvis. Its distribution, how- 

 ever, is essentially southern. 



Planorbis oregonensis Tryon is sinistral. The aper- 

 ture is greatly deflected on the lower extremity, a fact which 

 goes a great way toward placing it in the right position. 



Planorbis exacutus Say, is dextral. 



Planorbis parvus Say, is dextral. 



It is very difficult to experiment with these small and 

 fragile shells. Several specimens of exacutus I have been 

 able to examine with some degree of success, but, as a rule, 

 the results were not satisfactory. I am firmly convinced, 

 however, that the two species spoken of are dextral. 



Segmentina armigera Say, is dextral. It will be seen 

 from the above that the larger species of Planorbis are sinis- 



3 



