BAKER: FIELDBOOK OF ILLINOIS LAND SNAILS 33 



than that of today, since both the pawpaw and the osage orange 

 are found in geological deposits far to the north of their present 

 habitats. Each period was probably subjected to a warm, a cold 

 temperate and a boreal condition as the ice advanced and the 

 climate became arctic. The reverse condition prevailed when 

 the ice retreated and the climate became warmer. 



Effect of glacial periods.— The effect of this great field of 

 ice moving over the state was to kill all life that could not re- 

 treat in front of it. Such sedentary animals as snails could not 

 get away and must have been killed when the ice filled the valleys 

 and buried the life under the debris brought down. It has been 

 ascertained from a study of the snails in these deposits that the 

 ancient land snail fauna of the state was not exactly like that 

 living today. Members of some species, which differed in a 

 small way from their relatives now living, are known today as 

 races of these species. A few species became wholly extinct. 

 Others no longer live in Illinois but are now found far to the 

 west or north of the boundaries of this state. A few species once 

 common over the state are now found in northern Wisconsin 

 and Minnesota and in southern Canada. A deposit called loess, 

 formed centuries ago probably by the wind blowing fine dust 

 from the river bottoms, usually contains land snail fauna quite 

 unlike that of today. 



When the ice began to melt and retreat, and the climate 

 grew warm enough for plant life to become established again, the 

 land snail fauna once more took possession of the state. There 

 was migration from Indiana on the east, Missouri on the west 

 and Kentucky on the south. Since the retreat of the ice, some 

 species of land snails more or less foreign to these areas have 

 come into Illinois, along with the species which normally live 

 in this region, and are now widely distributed over the state. 

 The foreign species are found mostly in the river valleys border- 

 ing the southern and western parts ot the state. Some ot this 

 migration may have been geologically very recent. 



The Land Snail Fauna of Illinois 



All snails described in this fieldbook — in fact, all snails — 

 are included in the class Gastropoda. The following pages 

 describe 122 species and races of land snails found in Illinois. 

 Of these, all species but one belong to the order Pulmonata. 

 Six of the species described have been accidentally introduced 



