102 PROCEEDINGS OF THE ACADEMY OF [1881. 



rial, I have found much that is unsatisfactory, and it is not 

 asserting too much to say, that too many species have been made, 

 and that a careful revision is required ; and in this connection, 

 which shows the well-known variability of the group, I may men- 

 tion the examination of a parcel of specimens from a single " pond 

 back of Covington, Kentucky," kindly furnished by Professor 

 Wetherby, which admits of a separation resulting in three species 

 and a remainder which readily connects all three ; for this reason 

 I do not claim that the determinations as given are always con- 

 sistent, though carefully considered ; as before remarked, how- 

 ever, the point I have endeavored to present is not affected 

 thereby. 



II. On certain Aspects of Variation in American Planorbes. 



In the course of the preceding inquiry various aspects of varia- 

 tion, as exhibited in the material under examination were con- 

 stantly recurring. 



Without presuming to explain such phenomena, which would 

 quite likely be a difficult matter, even if all the peculiarities of 

 environment in each case, or of each lot of shells examined, were 

 known, and without such data, quite hypothetical, yet a few notes 

 and comments suggested by the forms referred to, may be worth 

 a passing notice. 



The larger so-called species of Planorbis may for convenience 

 in discussion be grouped as follows : 



First. Those in which the whorls are rounded ; that is to say 

 Figs. 16 and 17. if the tube or cone, as represented in the pre- 



ceding paper, was cut trans versel}^, the section 

 would show a rounded (not round) outline. 

 Examples— The typical P. corneus, L.,^ of 

 Europe; P. Guadaloupensis, Shj.f P. suh- 

 crenatus (figs. 16, It), Cpr.,^ and P. tumidus, 

 Pfr.,'* of Nicaragua, a quite persistent form, 

 not, however, quite as rounded as the others. 

 Second. Those in which the whorls are 

 either planulate, angulated, carinated or sub- 

 ^pTsubaenatus Cpr! carinatcd, which includes most of the larger 



1 Woodward's Manual, PI. XII, fig. 34 ; Sby.'s Manual, PI. XIV, fig. 

 311 ; Keeve, Conch. System., PI. CXC, fig. 1. 

 ■' Ibid, fig. 2. 

 ■' Binney, L. and F. W. Shells, N. A., Part II, figs. 176, 178. 



