490 limn^idj:. 



growth of this shell. Tlie following differences may be noted. It is 

 at least double, ofteji three times, the size. It is a thinner shell. 

 On the riuht side the revolutions are less compact, and exhibit a 

 larger portion of each whorl ; on the left side the suture is more and 

 the carina less distinct ; the aperture is much more expanded, and 

 projects far to each side of the preceding whorl. Inhabits the vicin- 

 ity of the Great Lakes. P. trivolvis differs from the next species, 

 P. lentus, by its carina, and the position of its aperture. 



[This species probably inhabits all of the United States and Can- 

 ada. It has been found from Fort Simpson to the Red River of Lou- 

 isiana, from Paget Sound to San Diego, in Utah, and from New 

 England through the Western and Middle States. Poey catalogues 

 it amono; the Cuban shells. 



*o 



Planorbis lentus. 



Fig. 132. 



Shell concave on both sides , whorls four, sub-carinate on the left side ; aper- 

 ture nowhere distinctly angular, right margin in the plane of that side. 



Planorbis lentus, Say, Amer. Conch, pi. 54, fig. 1. — Gould, Inv. 1st cd. 202, fig. 132. 



Shell orbicular, each whorl encircling the preceding, greenish 

 horn color at the circumference, yellowish at the sides and border- 

 ing the aperture ; on the right side concave, exhil^it- 

 ing scarcely three rounded volutions, separated by a 

 Avell-defined suture, and disappearing in a deep umbil- 

 icus ; left side presents a shallow cup, formed of four 

 compact, slightly carinated whorls, distinguished by a 

 toleraldy distinct suture ; surface marked with raised, 

 sub-equidistant lines of growth ; aperture large, ovate, 

 inclining to the right ; lip on the right side slightly 

 curved, lying in the plane of that side of the shell ; in 

 front, regularly and lu'oadly arched ; on the left side it stands out 

 considerably beyond the preceding whorl, and undergoes a sudden 

 curve before its junction with that wdiorl ; the lip is sharp, very 

 slightly spreading, and thickened within, by dark reddish-brown 

 callus. Greater diameter, seven tenths of an inch ; smaller diam- 

 eter, five twentieths of an inch. 



Animal dark olivaceous above and below ; foot oval, about one 

 lialf the diameter of the shell in length, minutely dotted beneath, 

 and frosted above with amber dots ; these arc abundant about the 



