612 MOLLUSCA 



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

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

 a])erture narrow, aljout half Ihc k'ligth of the shell: eircum- 

 polar; on muddy bottoms. 



Family 4. PLANOEBIDAE. 



Shell sinistral and discoidal, having a fiat or depressed 



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



hypnorum form; radula with biseuspid 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 Planorhidae here described : 



Oi Interior of whorls without teeth 1. Planorbis 



Oa Interior of whorls with teeth 2. Segmentina 



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 : 



Oi Shell sinistral and about 12 mm. wide or more. 



&i Aperture not flaring P. trivolvis 



5j Aperture flaring. 



Ci Keeled on both sides P. bicarinatus 



Cj Keel on one side P. campanulatus 



rtj Shell apparently dextral aiul about fi mm. wide. 



ftj Periphery acutely keeled P. exacutus 



6, Periphery rounded. 



Ci Aperture deflected upwards .P. deflectus 



Cj Shell hirsute P. hirsutus 



Ca 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, G mm. high and 12 mm. broad, keeled on both Pianorhix 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. 



