76 NEW-YORK FAUNA MOLLUSCA. 



GENUS PHYSA. Draparnaud. 



b 

 Animal oval, more or less spiral. Head with two long thread-like tentacles, with the eyes 



at their internal base. Mantle with two lobes, digitated on its margin, which can be re- 

 flected back so as to cover most of the shell. Foot long, rounded in front, pointed behind. 

 In other particulars resembling Limnea, except that the orifices are usually on the left. 

 Aquatic. Shell, often sinistral, oval, elongated or nearly globular, smooth, thin and fragile : 

 aperture oval, rounded in front, narrowed and subangular beneath ; pillar-lip somewhat 

 twisted, but without fold : spire more or less elongated, always prominent. 



Physa heterostropha. 



PLATE V. FIG, 82. 



(STATE COLLECTION.) 



Limnea heterostropha. Say, Nich. Encyclop. Vol. 4, pi. 1, fig. 6. 

 Physa id. Id. Jour. Acad. Nat. Sciences, Vol. 1, p. 172. 



P. id. Adams, Am. Journ. Science, Vol. 40, p. 268. 



P. id. Gould, Inrertebrata of Mass. p. 213, fig. 142. 



Description. Shell sinistral, subovate. Whorls four ; the first large ; the others small, 

 terminating rather abruptly in an acute apex. Surface smooth, but under the lens exhibits 

 very minute revolving and vertical lines : suture distinct. Aperture large, somewhat oval, 

 three-fourths the length of the shell, or rather more. Lip a little thickened on the inside in 

 adult animals. 



Color. Yellowish or greenish yellow r , becoming more dusky with age ; inside of the lip 

 dull reddish. Animal, olivaceous. 



Length, 0-5-0-7. 



A very common species in almost every pond and running stream. Often seen swimming 

 rapidly in a reversed position at the surface of the water. Infested by a parasitic Cercaria. 



Physa planorbula. 



PLATE V. FIG. 83. 



(STATE COLLECTION.) 

 Bulla fluviatilis ? Say, Journ. Acad. Nat. Sciences, Vol. 2, p. 178. 



Description. Shell small, thin and fragile, sinistral, cylindrical above, tapering beneath, 

 abruptly truncated on the summit; apex very slightly elevated above the truncation. Whorls 

 four ; the surface smooth, with minute revolving lines crossed by others equally minute. Body 

 whorl with an acute shoulder, the edge being slightly turned over. Aperture as long as the 

 shell, narrow above, dilated beneath, and broadly rounded. Outer lip acute, thin, and reflected 

 over the enlarged umbilicus. Color, light amber. Length, 0-2. 



