512 MOLLUSCA 



A. hypnorum (L.) (Physa elongata Say) (Fig. 781). Shell elongate, 

 with 6 whorls, yellow or brown in color, 17 mm. long and 8 mm. wide; 

 aperture narrow, about half the length of the shell : circuni- 

 polar; on muddy bottoms. 



FAMILY 4. PLANOEBIDAE. 



Shell sinistral and discoidal, having a flat or depressed 



Fig. 781 spire, in some cases apparently dextral ; tentacles fili- 



Aplexd 

 hypnorum form; radula with biscuspid central teeth; genital pores 



on the left side; jaw composed of 3 pieces: 170 species; in 

 streams, ponds, and lakes, on muddy bottoms or on submerged objects 

 or vegetation. 



Key to the genera of Planorbidae here described: 

 o t Interior of whorls without teeth 1. PLANORBIS 



1. PLANORBIS L. Shell discoidal and sinistral, the spire being flat- 

 tened or sunken, with 3 to 5 whorls; umbilicus large and wide; lip simple 

 and acute: over 150 species; cosmopolitan, but found mostly in temperate 

 regions ; 26 American species. 



Key to the species Planorbis here described : 



G! Shell sinistral and about 12 mm. wide or more. 



6 t Aperture not flaring P. TBIVOLVIS 



ft, Aperture flaring. 



c t Keeled on both sides P. BICABINATUS 



c a Keel on one side P. CAMPANULATUS 



ffi 2 Shell apparently dextral and about G mm. wide. 



6 a Periphery acutely keeled P. EXACUTUS 



6 a Periphery rounded. 



G! Aperture deflected upwards P. DEFLECTUS 



c a Shell hirsute P. HIRSUTUS 



fy Shell regular and smooth P. PARVUS 



P. trivolvis Say (Fig. 782). Shell sinistral, yellowish or brown, 

 with 4 whorls; spire sunken; aperture large, with a sharp and simple 

 lip, which has a V-shaped angle above; umbilicus 

 showing almost all of the whorls; height 9 mm.; 

 width 22 mm. : North America ; the commonest species. 



P. bicarinatus Say. Shell sinistral, deeply con- 

 cave on both sides, pale yellow or brown, with 3 



Fig. 782 



whorls, 6 mm. high and 12 mm. broad, keeled on both Planorbis trivolvis 



(Baker). 



sides ; aperture large : northern and central states ; A, upper surface ; 



. B, under surface, 



south to Georgia and Kansas; California. 



P. campanulatus Say. Shell sinistral, flat, with 4 whorls; aperture 

 flaring and bell-shaped; height 7 mm.; breadth 14 mm.: northern and 

 central states. 



