LIMNEANA. MOLLUSCA. PLANORBIS. 201 



Planorbis trivolvis. 



Shell concave on both sides ; whorls four, strongly carinated on 

 the left side ; aperture acutely angulated by the carina, right mar- 

 gin extending beyond the plane of that side. 



Figure 131. 



State Coll., No. 94. Soc. Cab., No. 1275. 



Cochlea triura orbium, Lister; Conch., pi. 140, f. 46. Petiver; Gazophyl, 



pi. 106, f. 17. 

 Planorbis trivolvis, Say ; JVicliotson s Enryc, {Amer. ed.,) iv. pi. 2, f. 2. Amer. 



Conch., pi. 54, f. 2. 



Shell orbicular, yellowish-white, brownish, or chestnut-color ; 

 umbilicated on the right side, cup-shaped on the left ; on the right 

 side scarcely three volutions, separated by a profound suture, are 

 visible, as they disappear in the umbilicus, their faces, especially 

 those of the interior whorls, being slightly carinated ; on the left 

 side at least four whorls are seen, which, by their faces, form a 

 cup-shaped depression, scarcely distinguished by the suture, 

 except the last half of the outer whorl, on the whole of which a 

 well-marked carina revolves, forming a margin to the cup ; the 

 carina gives the whorl a flattened appearance on this side ; surface 

 covered with fine, regular, raised, transverse lines, somewhat 

 grooved between them ; aperture sub-ovate, inclining to the right, 

 its right margin more advanced than the left, broadly and regularly 

 rounded ; left lip abruptly angulated where the carina terminates ; 

 lip usually thickened within, and of a reddish brown-color. Large 

 diameter y\ inch, small diameter y\ inch. 



Jlnimal dark-russet or dusky, covered with pale-yellowish dots. 



Found in the western parts of this State in rivers and ponds. 

 It is widely extended over the Northern and Western States. 



Planorbis corpuUntus of Say seems little else than an exuberant 

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

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

 the right 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 



2^ 



