FIELD BOOK OF PONDS AND STREAMS 



Fig. 250. — TrivoJvis parvus: i, shell coils; 2, shell 

 showing body aperture. 



Trivolvis parvus (Graulus parvus of Baker). — Its shell (Fig. 

 250) is small, only one-fifth of an inch wide, with 3 or 4 rapid- 

 ly enlarging coils. These very small wheel-snails are common 

 in mats of filamentous alg^ (p. 50), on pond weeds and 

 Myriophylliim, on most thick submerged vegetation. 



Fig. 251. — Hairy wheel-snail, Planorhis liirsiitus: 

 I and 2, shell showing coils; 3, shell showing body 

 aperture. 



Hairy wheel-snail, Planorbis hirsutus (Graulus hirsutus of 



Baker).— These snails (Fig. 251) are found in still water of 

 ditches and swamps. They are sometimes distinctly recog- 

 nizable by the delicate hair-like projections of the outer shell 

 layer; these disappear when the outer layer is worn off. Shell 

 small, about one-fifth of an inch. Found in the eastern and 

 northern United States. 



Fig. 252. — Limpet, Ancylus (Ferrissia of Baker); 

 underside of the animal showing its gill. 



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