362 THE CHICAGO ACADEMY OF SCIENCES. 



horn or light green, showing former peristome marks as in 

 decisum; surface polished, smooth save for the somewhat ele- 

 vated, crowded growth-lines which roughen the shell to some 

 extent; the shell beneath the epidermis is chalky white; apex 

 small, similar to decisum, but more prominent; spire elevated, 

 elongated, conic; sutures impressed, almost channeled; whorls 

 six to seven in fully adult specimens, rather flat-sided, sloping 

 near the sutures, where they are almost angular; the whorl oc- 

 cupies three-fourths of the entire length of the shell, and the 

 periphery is perceptibly flattened; aperture broadly ovate, 

 rounded below and contracted above; forming a sort of "bay" 

 at the columella, white or bluish-white within; peristome thin f 

 acute, simple, the terminations connected by a rather heavy 

 columellar callus which is reflected over the umbilicus; the 

 parietal callus is bluish-white and is sometimes bordered by a 

 black stripe; base rounded, imperforate, somewhat wrinkled. 

 Operculum similar to that of decisum. 



Length, 32.00; width, 20.00; aper. length, 17.00; width, 11.00 mill. (10113.) 



31.00; " 19.00; " " 16.50; " 11.00 " (10113.) 



29.00; " 17.50; " " 15.00; " 10.00 " (8857.) 



30.00; " 18.00; " " 15.50; " 10.00 " (8844.) 



40.00; " 23.00; " " 21.00; " 13.50 " (9343.) 



Animal: Similar to decisum. 



Radula: Same as that of decisum. 



Distribution: Massachusetts to Illinois and Michigan, 

 south to Central Ohio. 



Geological distribution: Pleistocene; Loess. 



Habitat: Similar to decisum. 



Remarks: This species is almost always confounded with 

 decisum. It is a more heavy shell, the spire is more produced, 

 the aperture is more sigmoid and the whorls are more flat- 

 sided. The two species are almost always found associated 

 together and it is a question whether they do not interbreed. 

 It is certainly very difficult to separate some of the forms of 

 these two species. This is the most common Campeloma in 

 this region and grows to a large size. In Wolf Lake it is very 

 plentiful, and in the North Branch of the Chicago River it is 

 quite common and very typical. It is universally distributed 

 and has been found fossil on the old lake beaches and by Mr. 

 Jensen in sand banks on the lake shore north of Graceland 

 avenue. 



