GASTROPODA 513 



P. exacutus Say (Fig. 783). Shell apparently dextral, with an acute 

 periphery, 4 whorls, white or light yellowish in color; 6 mm. wide: 

 northern 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. 



Fig. 783 Fig. 784 Fig. 785 



Fig. 783 PlanorMs exacutus (Baker). Fig. 784 PlanorUs deflectus (Baker). 

 Fig. 785 PlanorMs parvus (Baker). 



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

 with revolving lines of delicate hairs; aperture large and very oblique; 

 diameter 5 mm.: circumpolar; south to Washing-ton. 



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. 



2. SEGMENTINA Fleming. Shell sinistral, discoidal, 

 with a flattened or sunken spire, furnished internally 

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



QF1fl>'l(jTQ> (i->3.kGr). 



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. 



ANCYLUS Geoffroy. River limpets. With the characters 

 above given: 16 American species. 



A. parallelus Haldeman. Shell elongate, with the sides 

 Fig. 787 parallel and the ends blunt, with apex nearly in the center, 

 rivuiarte 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. 



