1881.] 



NATURAL SCIENCES OF PHILADELPHIA. 



103 



Xorth American species 



riGS. 18 and V.>. 



Examples — P. corpidentus (figs. 18, 19), 

 Saj^,' P.Traskii^ Lea., P, occiden- 

 talism Cp., and P. hicarinatus 

 (fig. 20), Sa3\ In these the tube, 

 if cut transversely, would present 

 an outline more or less angulated. 

 Forms like P. trivolvis (fig. 21), 

 Say, connect the two groups ; for 

 while in some inst'^nees this 

 species exhibits the rounded 

 whorls of the first, it imperceptibh" differenti- 

 ates from the above to obtuse angulation, and 

 p. corpuientus, Say. ^^^^^^q to the subcarinate forms of the second 



bicarinatiLS, 

 Say. 



jronp, 

 P. amnion (fig 



Fig. 21. 



Fig. 22. 



P. irivolvis. Say. 



22), Gould, must be mentioned here, as it illus- 

 trates another aspect of 



variation, that of a more 



rapid enlargement of the 



whorls, the result of a more 



obtuse cone than in tri- 



volvis; this, when flattened 



above or angulated, gives 



us the form P, Traskii, the 



most striking of all the 



American Planorbes ; it is 



the extreme or culmination 



of the flattened or planu- 

 lated aspect in the second group of species, 

 of which P. corpidentus is a well known form 

 and more widelj- distributed than the other ; 

 Dr. Cooper's P. occidentalis being an inter- 

 mediate link between t^'pical P. tricoluis and 

 ordinary average specimens of P. corpidentus. 

 Southern specimens of/*, trivolvis seem to 

 be nearer the southern form of P. lentus than 

 do average specimens of these alleged species from northern 

 stations ; and both of the above from soutlierl}- stations approach 

 more closel}- to the European corneus than do northern specimens 

 of the same ; the same mav be said of the Xicarasfuan P. tumidus. 



p. ammon, G-IJ. 



^ Biiiney"s figures, ibid. 



