GASTROPODA 5I3 



P. exacutus Ray (Fi- 783). Shell apparently dextral, with an aenfo 

 periphery, 4 whorls, while or li-hi yellowish in color; (5 nnn. wide: 

 noiihern states, south to Kansas. 



P. deflectus Say (Fig-. 784). Shell apparently dextral, with a blunt 

 keel at the periphery, and 4 whorls, the largest whorl being deflected 

 from the plane of the others: North America, south to Washington. 



^S> c^2Z^ c^^o 



Fig. 783 Fig. 784 Fig. 785 



Fig. 783 — Planorhis exacutus (Baker). Fig. 784 — Planorhis deflectus (Baiter). 

 Fig. 785 — Planorhis parvus (Baker). 



P. hirsutus Gould. Shell brownish, with 3 whorls; surface beset 

 with revolving lines of delicate hairs; aperture larg-e and very oblique; 

 diameter 5 mm.: circumpolar; south to Washington. 



P. parvus Say (Fig-. 785). Shell apparently dextral, with a rounded 

 periphery and yellowish to blackish in color, with 4 whorls; 5 mm. in 

 diameter ; periphery rounded : North America ; very 

 common, especially on water plants. /^<3P) 



2. Segmentina Fleming. Shell sinistral, discoidal, d^^^'KS^ 



with a flattened or sunken spire, furnished internally „ 



with transverse partitions or teeth ; lip simple : but few Segmentina 



^ 7 jr ir armigera (Baker). 



species, principally in the Old World. 



S. armigera (Say) (Fig. 786). Shell flat on the upper and concave 

 on the lower side, with 4 whorls; 5 internal teeth present, far within 

 the aperture, which nearly close the passage; diameter 6 mm.; height 

 2.5 mm.: North America; often abundant; sometimes gregarious. 



Family 5. ANCYLIDAE. 



Shell thin, non-spiral, conical, with the apex median or directed to 

 the right, and an oval aperture; jaws 3, covered by papillae; tentacles 

 triangular, with eyes at their base: 8 genera and about 60 

 species; cosmopolitan. 



Ancyl¥S Geoffroy. River limpets. With the characters 

 above given : 16 American species. 



A. parallelus Haldeman. Shell elongate, with the sides 



parallel and the ends blunt, with apex nearly in the center, 



dark green in color; 5 mm. long and half as wide: New 



England, on stones and leaves in brooks and ponds. 



A. rivularis Say (Fig. 787). Shell oval, horny, white within, 6 



mm. long; apex obtuse, nearer to and leaning towards one side and one 



end: eastern states, sometimes common on dead shells and stones. 



