202 INVERTEBRATA OF MASSACHUSETTS. 



far to each side of the preceding whorl. Inhabits the vicinity of the 

 Great Lakes. P. trivolvis differs from the next species by its carina, 

 and the position of its aperture. 



Planurbis lentus. 



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

 left side ; aperture nowhere distinctly angular, right margin in 

 the plane of that side. 



Figure 132. 



State Coll., No. 83. Soc. Cab., No. 1270. 



Planorbis lentus, Say; Jimer. Conch , pi. 54, f. 1. 



Shell orbicular, each whorl encircling the preceding, greenish 

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

 dering the aperture ; on the right side concave, exhibiting scarcely 

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

 disappearing in a deep umbilicus ; left side presents a shallow 

 cup, formed of four compact, slightly carinated whorls, dis- 

 tinguished by a tolerably distinct suture ; surface marked with 

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

 clining 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 di- 

 ameter j\ inch, smaller diameter 2% inch. 



Jlninial 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 

 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 distinguish- 

 ed 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 



