FAMILY LIMNIADwE PLANORBIS. 59 



FAMILY LIMNIADJB. 



Shell always complete, thin, smooth, much convoluted: outer lip trenchant, not reflexed. 

 Animal with its body elongated, distinct from the foot : no cuirass, but a collar formed 

 around the neck by the margin of the mouth. Head furnished with a wide sort of veil. 

 Tentacles two, with the eyes at their base. Pulmonary orifice on the collar. Organs of 

 generation separated. Vent near the pulmonary orifice. All fluviatile. 



GENUS PLANORBIS. Lamarck. 



Shell discoidal, sinistral. Spire depressed or concave, exhibiting the whorls above and below. 

 Aperture broader than long ; the margin sharp, and not reflexed. Animal elongated, com- 

 pressed, with two very long filiform tentacles. Mouth with a crescent-shaped tooth above, 

 and the tongue armed with small hooks, surmounted by a sort of short emarginated veil. 

 Breathing-hole dextral, on the collar, and the vent near it. Organs of generation on the 

 same side, separate ; the male near the tentacle, and the other at the base of the collar. 

 All living in fresh water. 



Planorbis trivolvis. 



PLATE IV. FIG. 99. a. i. 



(STATE COLLECTION.) 



Planorbis trivolvis. Sat, Encyclop. Nich. Am. ed. Vol. 4, pi. 2, fig. 2. 

 P. id. Say, American Conchology, pi. 54, fig. 2. 



P. id. Gould, Invertebrate of Mass. p. 201, rig. 131. 



Description. Shell discoidal. Whorls three or four, marked with regular transverse lines, 

 rather acutely carinated above and beneath, more obtusely so on the circumference : these 

 carinae most obvious on the young shell. Suture most apparent on the upper or right side, 

 which has a depressed spire ; beneath cup-shaped. Aperture large, higher than wide, 

 embracing a considerable portion of the body-whorl, inclining to the left. Lip abruptly 

 angulated at the termination of the carina, thickened within. 



Color. Pale yellow or olive. Animal dusky, with pale yellowish confluent spots. 



Diameter of the shell, 0*5 - 0-7 ; height, 0-2-0-3. 



This species, which ranges through the Northern and Western States, is abundant in many 

 of the streams and ponds of New- York. 



8* 



