Occasional Papers of the Museum of Zoology 35- 



rivers were heavy shells with distinct growth lines, and were 

 variable in shape and texture, although some approximated 

 typical shells (No. 4 and No. 8). Those from the Upper 

 Twin Falls of the Menominee (No. 2 and No, 3) were large, 

 but quite light, and were not so deeply marked by the growth 

 lines as were the heavier shells from the Sturgeon (Nos. 4-8). 

 The shells from both of these habitats were apt to have the 

 carinae much more distinct than in typical shells, and the lower 

 portion of the apertures were produced and sharply angulated, 

 somewhat after the manner of P. antrosus portagensis 

 (Baker). Some even had the aperture elongated, both above 

 and below, so as to more closely resemble that form in general 

 shape. Table VI shows the measurements of some of these 

 shells. These variations were perhaps caused by the direct 

 efifect of the physical environment: swift current, an abun- 

 dance of food and lime, and the shape of the rocks to which 

 they clung. 



Tabi.e VI. Measurements of P. antrosus 



lom, II, 14 , 27, 28. Preeminently the variety from the slug- 

 gish water habitats where the bottom was sandy or mucky. 

 Shells light-colored and fragile ; beautifully striated. Beaver 

 pond specimen : g. diam. 14 mm. ; 1. diam. 11.2 mm. ; alt. apert. 

 7.3 mm. ; alt. opposite apert. 5.8 mm. ; alt. at inside of apert. 

 4.2 mm. 



