488 LIMNiEID^. 



which are, in truth, little parasites (^Gordius inquilinus, Miill.) at- 

 tached like leeches, and which derive their nourishment from the 

 fluids of the animal, without his having the power to dislodge them. 

 [From Kansas to the District of Columbia, and from the Atlantic 

 to the Pacific in the British possessions, ranging as far north as 

 Russian America. It is one of the species common to the three con- 

 tinents. I consider it identical with the B. hypnorum. 



Genus PL,A]\ORBlS, Guettard. 1756. 



Tentacles slender, filiform. Foot short, ovate. 

 Shell dextral, discoidal ; spire depressed, whorls 

 numerous, visible on both sides ; aperture crescentic, 

 or transversely oval ; peristome thin, incomplete, the 

 upper margin produced. 

 Animal of P. inrsit- Jaw siuglc, supcrioF, arclicd. 



Lingual membrane short, with broad, stout teeth ; 

 apices recurved and prolonged into tusk-like tubercles. 



The genus Planorbis is widely distributed over the globe, but 

 usually prefers the more temperate regions. It is found in every 

 part of this continent, reaching into Mexico, and apparently much 

 more abundant there than the other genera of the family. 



Most of the sections or sub-genera are represented in North Amer- 

 ica. The South American Taphius is most nearly allied to the Car- 

 inifex of the Pacific coast. 



The name Planorbis is now universally applied to the genus. 



The species of this genus have a dextral shell, but the orifices of 

 the generative, excretory, and respiratory organs are on the left of 

 the animal, as in Physa. They are sluggish in their habits, prefer- 

 ring stagnant pools. 



Say considered the shells sinistral, a fact which must be borne in 

 mind while studying his descriptions. 



Planorbis trivolvis. 



Fig. 131. 



Shell concave on both sides ; whorls four, strongly carinated on the left side ; 

 aperture acutely angulated by the carina, right margin extending beyond the plane 

 .of that side. 



Planorbis trivolvis, Say, Nich. Eiicyc. pi. 2. fig. 2 (1817, 1818, 1819) ; Am. Conch, part 6, 

 pi. 54, fig. 2 (1834) ; Binnev's cd. 44, pi. 70, fig. 2 ; pi. 54, fig. 2. -De Kay, N. 



