SUBALPINE MOLLUSCA. 89 



culiar to P. trivolvis. Then, adopting the subgenus 

 HeUsonia Swainson, a new species is given as P. (H.) 

 occidentalis J. G. Cooper, with "whorls five or six, 

 nearly all visible- above, much narrowed and subangled 

 below, nearly smooth, mouth little higher than wide, di- 

 ameter 0.80 to 1. 12, altitude 0.50 to 0.70." While this 

 was unlike any described form, and had the somewhat 

 enlarged outer whorl of HeUsonia, manv intermediate 

 specimens now prove that it is only the mature form of 

 P. til mens. Its last whorl is not really more dispropor- 

 tioned than that of a very large P. corneus, and, together 

 with some other species, it shows that subg. Helisoma is 

 untenable. Its nearest Eastern analogue is P. lentits 

 Say, and that has been by many considered only a vari- 

 ety of Helisoma trivolvis. The figures given show ours 

 to have the form of a non-carinate species. We consider 

 the carination, first used by Say as a specific character, 

 more important for the grouping than the later subgeneric 

 divisions, and it also serves in many cases to determine 

 young shells, which have been often described as new 

 species. 



It is used also as an important character in the differ- 

 entiation of Menetus, Gyraultis, etc., being in them mar- 

 ginal instead of lateral, and while such forms are of very 

 ancient date geologically, the lateral carina seem to ap- 

 pear only in the recent period. The maximum of cari- 

 nation is reached in the allied genus Carinifex, only 

 known in late tertiary and recent epochs. A rounded 

 or blunt angle is not intended by the word carina, which 

 must be sharp. 



One reason given for supposing immature shells 

 to be good species is that they are found in springs and 

 ponds without larger ones. It is yet to be determined 



2d Ser., Vol. III. ( 8 ) August 8, 1890. 



