CHAPTER XLII: THE VALVE SHELLS 



Family VALVATiDiE 



Shell depressed, often almost discoidal, umbilicated; oper- 

 culum round; epidermis green. Animal with long snout and 

 tentacles; foot cleft in front; branchial plume long, branched, 

 thrust partially out when the mollusk is walking. Teeth of radula 

 in seven series, broad, hooked. Fresh water or terrestrial mol- 

 lusks. 



Genus VALVATA, Miill. 



Characters of the family. Small, thin, flat-coiled shells, 

 found in slow-running brooks and ditches, or ponds, in Europe 

 and North America. The eggs are laid in a single globular capsule. 

 The capsules are fastened to pebbles or stems of plants. 



The Three-keeled Valve Shell (F. tricarinata, Say) exhibits 

 three coils, each of which bears a keeled shoulder. The mouth 

 is round, expanding like a bell; the operculum is spiral with a 

 central nucleus. When progressing, the gill plume is lifted above 

 the head. Diameter, \ inch. 



Habitat. — Delaware River. 



V. sincera, Say, similar in most particulars to the preceding, 

 has rounded whorls. The pit is larger. 



Habitat. — Northwest Territory. 



The Green Valve Shell (F. virens, Tryon), a minute, 

 swollen top shell, bright green, is \ inch in diameter. 



Habitat. — Gear Lake, Cal. 



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