GASTROPODA 511 



Key to the genera of Physidae here described : 



Oj Margin of mantle digitate 1. PHTSA 



<z a Margin of mantle simple 2. APLEXA 



1. PHYSA* Draparnaud. Mantle reflected over a portion of the 

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

 Key to the species of Physa here described : 



! Whorls 5 or 6. 



bi Aperture three-fourths the length of shell or more P. SAYI 



63 Aperture two-thirds the length of shell or less P. GYBINA 



a, Whorls about 4. 



6 X Spire very short P. ANCILLARIA 



6 a Spire acute P. HETEROSTKOPHA 



P. sayi Tappan. Shell with 5 or S 1 /^ whorls, with an elevated, acute 

 spire and w r ith 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 heterostropha (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.: Mississippi 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-fourths 

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

 mm.; breadth 8.5 mm.: entire country; 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 Fhysae," by O. A. CrandaH, Nautilus, 

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



