GASTBOPODA 511 



Key to the genera of Physidae here described : 



fli Margin of inantlo digitate 1. Piiysa 



a. Margin of mautlo simplo 2. Aplexa 



1. Physa* Draparnaiul. Mantle refiected over a portion of the 

 shell and fringed with finger-like tilaments: 100 species. 

 Key to the species of Physa here described : 



Oj Whorls 5 or 6. 



hi Aperture three-fourths the length of shell or more P. sayi 



6j Aperture two-thirds the length of shell or less P. gyrina 



rtj Whorls about 4. 



6i Spire very short P. ancillaria 



&j Spire acute P. hetekostropiia 



P. sayi Tappan. Shell with 5 or 5y2 whorls, with an elevated, acute 

 spire and with numerous impressed spiral lines; aperture large, three- 

 fourths or four-fifths the length of the shell; length 22 mm.; width 13.5 

 mm. : northern states, west to Rocky Mountains, south to Ohio River. 



Fig. 779 Fig. 780 



Fig. 779 — Physa gyrina (Baker). Fig. 780 — Physa Jieterostropha (Baker). 



P. gyrina Say (Fig. 779). Shell an elongate cone with 5 or 6 whorls, 

 with numerous impressed spiral lines; aperture more than half but less 

 than two-thirds the length of the shell ; length 23 mm. : IVIississippi valley, 

 the commonest species in the central states. 



P. ancillaria Say. Shell with 4 or 5 whorls, a very short spire, and a 

 very large bulging lower whorl; aperture almost as long as the shell; 

 color pinkish or yellowish ; length 14 mm. ; suture inconspicuous : eastern 

 and northern states, often common. 



P. heterostropha (Say) (Fig. 780). Shell smooth and polished and 

 without spiral lines, with 4 whorls, the first very large, the others very 

 small, with an acute apex; aperture large, elongate, and three-fourth^ 

 the length of the shell; color yellowish-brown or blackish; length 14 

 mm.; breadth 8.5 mm.: entire countiy; the commonest species in the 

 eastern states. 



2. Aplexa Fleming. Mantle not reflected over the shell and with 

 a simple edge: several species. 



* See "A Revision of the Physae of North Eastern Illinois," by F. C. Baker, 

 Nautilus, Vol. 14, p. 16, 1900. "The American Physae," by O. A. Crandall, Nautilus, 

 Vol. 15, Nos. 3, 4, 5, 6, 1901. 



