PLEISTOCENE MOLLUSCA FROM NORTHWESTERN 

 AND CENTRAL ILLINOIS^ 



FRANK COLLINS BAKER 

 Museum of Natural History, University of Illinois 



Two very interesting and important collections have recently 

 been submitted for study by members of the Illinois State Geo- 

 logical Survey. They include several new forms, besides a number 

 of species not before recorded from Illinois Pleistocene deposits, 

 though several of these have been known from Iowa glacial deposits 

 for many years. 



In the volume on the Life of the Pleistocene (Baker, 1920&, p. 368) 

 the author listed as extinct six species of mollusks. That so small 

 a number of species in so large a group should have become extinct is 

 noteworthy, but is paralleled by the plants of which seven species 

 are extinct. It is believed by the writer that careful analysis of 

 Pleistocene Mollusca will show that more species or varieties have 

 become extinct than is indicated by present records. Many 

 species have been linked with existing forms though they vary 

 quite enough to constitute distinct species or races. It will be 

 found that some species are abundant in Pleistocene deposits, but 

 rare living, indicating that the species is approaching extinction. 



In the study of Pleistocene fossils it is of the greatest importance 

 that minute differences between species be recorded, for it fre- 

 quently happens that two deposits contain species closely related 

 but yet sufficiently different to indicate a difference in climate, 

 habitat, or general environment. A case in point is Succinea avara 

 and Succinea vermeta, two closely related species or races, the latter 

 of which is abundant in loess deposits and other Pleistocene beds. 

 The so-called "lumper" of species can do great harm to the study 

 of fossil faunas, whereas the so-called "splitter," recognizing small 

 differences, may be of real help in the discrimination of faunas and 



' Contribution from the Museum of Natural History, University of Illinois, No. 20. 



.43 



