20 PLANORBIS CORPULENTUS. 



DESCRIPTION. 



Shell large, composed of four or five strap- 

 shaped, compact whirls, having a considerable 

 transverse diameter (from carina to carina) 

 which much exceeds the diameter of the aper- 

 ture, taken at right angles to the axis: the lines 

 of growth are distant, elevated, and conspicu- 

 ous, giving the surface a rough appearance: 

 the right side is widely and deeply umbilicated, 

 and has a strong tendency to obtuse carination; 

 the left side is carinated, and presents a wide, 

 and rather deep cup: the aperture has but little 

 obliquity, extends considerably to the right and 

 left beyond the penultimate whirl, and has the 

 labium expanded. 



Color light ochraceous, the aperture some- 

 times presenting a zone of chesnut. 



Geographical Distribution. Common in 

 Winnipeek river and lake. Lake of the Woods, 

 and Rainy lake. — Say. Lewis' river. — Mr. 

 Nuttall, (specimen figured.) 



OBSERVATIONS. 



The distinguishing character of this species, 

 is the width and flatness of the whirls, in a 



