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 



