GASTROPODA 543 



P. lapidaria (Say) (Fig. 844). Shell slender, dark brown in color, 

 and shining, with 6 whorls, 6.5 mm. long and 3 mm. wide: eastern and 

 central states, under stones on the margin of streams or in moist places 

 on land or on aquatic plants; common. 



P. cincinnatiensis (Anthony). Shell robust, with 4 or 

 5 whorls, greenish or brown in color, and shining, 4.5 mm. 

 long and 2.3 mm. wide: central states, in moist places on 

 Fig. 844 land, or on stones in the water. 



lapidaria^ P. californlca Pilsbry. Shell thin, brown in color, 5 



mm. long : San Francisco and Oakland. 

 6. Palttdestrina D^Orbigny. Shell small, elongate, conical, with 

 an elevated spire; apex generally obtuse; aperture ovate; lip thickened; 

 operculum horny: in fresh water. 



P. nickliniana (Lea) (Fig. 845). Shell with 6 whorls, greenish in 

 color, shining, 4.5 mm. long, 2 mm. wide; umbilicus indented: eastern 

 and central states; often common on aquatic vegetation in ponds and 

 rivers. 



Family 6. SKENEIDAE. 



Shell small, with a very low spire, being almost discoidal, and a 

 wide umbilicus ; operculum horny, multispiral : 1 genus, in northern seas. 



Skenea Fleming. With the characters of the family: 1 American 

 species. 



^''^ 



Fig. 845 Fig. 846 Fig. 847 



Fig. 845 — Paludestrina nickliniana (Baker). Fig. 846 — Stkenea planorbis (Verrill). 

 Fig, 847 — Vivipara contectoides (Baker). 



S. planorbis (Fabricius) (Fig. 846). Shell minute, flat, with 3 

 whorls, yellowish, smooth; aperture circular; length .8 mm.; width 1.3 

 mm. : Long Island Sound to Greenland ; ^common under stones near low- 

 water mark; Pacific coast; Europe. 



Family 7. VIVIPAEIDAE. (Paludinidae.) 



Shell conical or globose, with rounded whorls and an olive-green 

 periostracum ; aperture simple, roundish; operculum horny and usually 

 not spiral; animal with a prominent, broad snout, and eyes on short 

 stalks at the outer base of the stout tentacles; viviparous, the young 

 remaining in the parent shell for some time after hatching: about 6 

 genera and over 200 species; in fresh water in all parts of the world. 



