PLEISTOCENE MOLLUSCA FROM ILLINOIS 6i 



Planorhis exacuous Say. A single specimen of this characteris- 

 tic species was observed. It differs from Recent exacuous in having 

 a bluntly acute periphery, hence higher whorls, and in the decided 

 deflection of the aperture. It may be an individual variation, as 

 but the one specimen was found. 



Planorhis altissimus Baker. This common small Planorhis 

 occurred abundantly in the deposit. It is the most abundant 

 species of the genus in Pleistocene deposits. 



Calha obrussa decampi (Streng). Common, but most of the 

 material is immature. 



Mr. Culver also collected a number of shells in Section 23 of 

 the same township, in a slight depression which drains fairly well 

 to the southwest. Mr. Culver says of this material: 



The shells lie scattered through about a foot of black mucky soil, which 

 overlies clay which is presumably weathered Marseilles till. Although lying 

 at an elevation considerably above the present alluvial flats of the Illinois 

 River, this appears to be merely an accumulation of recent muds, and hence 

 cannot be referred to the glacial epoch. 



Four species are represented in the lot: 



Galba reflexa Say. Planorhis trivohis Say. 



Physa gyrina Say. Planorhis pseudotrivolvis Baker. 



All were abundant except Planorhis pseudotrivolvis, of which but 

 one specimen was present. This is an interesting find, because 

 this form of Planorhis is present in the Chicago deposits and is now 

 living near Urbana (see Baker, 1920, p. 123). The fauna is one 

 that we now find in swales or summer-dry ponds in many parts of 

 Illinois, or in the small branches of rivers near their sources. 



LIST OF WORKS CITED 



Baker, Frank Collins. 



1918. " Post-glacial Mollusca from the Marls of Central Illinois," Journal 

 of Geology, Vol. XXVI, pp. 659-71. 



1919. "Description of a New Species and Variety of Planorhis from 

 Post-glacial Deposits," Nautilus, Vol. XXXII, pp. 94-97- 



1920. "A New Planorhis from Illinois," Nautilus, Vol. XXXIII, pp. 1 23- 



25- 



1920a. "Pleistocene Mollusca from Indiana and Ohio," Journal of Geology, 

 Vol. XXVIII, pp. 439-57- 



