Fresh- Water Shells. 203 



Inliabits the vicinity of the Great Lakes. P. trivolvis differs 

 from its next species by its carina, and the posit-on of its aper- 

 ture. 



Planoreis Lentus. (Say.) 



Shell orbicular, each whorl encircling 

 the preceding, greenish horn-colour at 

 the circumference, yellowish at the sides, 

 and bordering the aperture ; on the right 

 side concave, exhibiting scarcely three 

 rounded volutions, separated by a well-defined suture, and disap- 

 pearing in a deep umbilius ; 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 

 whorl ; the lip is sharp, very slightly spreading, and thickened 

 within by dark reddish-brown callus. Greater diameter -^-^ inch, 

 smaller diameter -£-^ inch. 



Animal dark olivaceous above and below ; foot oval, about 

 one half the diameter of the shell in length, minutely dotted be- 

 neath, and frosted above with amber dots ; these are abundant 

 about the bases of the tentacula ; edges of mouth honey-yellow ; 

 motions sluggish. 



Found abundantly in all our ponds, small brooks, and stagnant 

 pools. 



This is a somewhat darker shell than P. trivolvis, and is dis- 

 tinguished from it by its left side and its aperture. The cup of 

 the left side is less smooth and regular, and is not bounded by the 

 sharp, elevated line ; when this shell is laid upon its right or upper 

 side, the lip of that side will scarcely touch the plane on which it 

 lies, while, in P. trivolvis, the shell would be lifted by the lip ; the 

 aperture has not the sharp angle of the left side, produced by the 

 termination of the carina, but in the young stages it is difficult to 

 distinguish the two. It is very closely allied to P. corneus of 

 Europe ; but in that shell the left side is scarcely concave, and the 

 suture is deep ; the aperture is nearly orbicular, being almost 

 equally rounded on both sides. 



