490 limn^idj:. 



growth of this slicll. The following differences may he noted. It is 

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

 On the ri_i2,ht 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 Puget Sound to San Diego, in Utah, and from New 

 England through the Western and Middle States. Poey catalogues 

 it among the Cuban shells. 



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 ed. 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, exhibit- 

 ing scarcely three rounded volutions, separated by a 

 well-defined suture, and disapjiearing in a deep umbil- 

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

 compact, slightly carinated whorls, distinguished by a 

 tolerably 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 broadly arched ; on the left side it stands out 

 considerably beyond the preceding whorl, and undergoes a sudden 

 curve before its junction with that wliorl ; 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 

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

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



