LYMNyEID.E OF NORTH AMERICA. 67 



humilis rustica. apicina. 



obrussa. 



galbana. 



palustris. 



It is noteworthy that the species living in tropical and semi-tropical 

 regions are more uniform in size, shape and sculpture than are those 

 species living in northern and colder climates, where variation is 

 marked and apparently endless. The family is characteristic of boreal 

 and temporate regions and the number of species grow less toward 

 the warm climate of the south and the individuals become smaller. 



B. THE GLACIAL PERIOD. 



The Glacial Period, with its successive stages of extended gla- 

 ciation, undoubtedly caused the absolute extinction of all molluscan 

 life in that area occupied by the great ice sheet. The Lymnseid fauna 

 must have been gradually driven south until it occupied the territory 

 south of the Ohio and Missouri rivers (figure 2). A single ice sheet 

 invaded the northern Mississippi Valley region as far south as southern 

 Illinois, driving the Lymnseas into the lower Mississippi drainage. Just 

 what effect the driftless area in Wisconsin may have had on the fauna 

 is not known, but as it is believed that the ice failed to cover this region, 

 which was several thousand square miles in extent, and as there must 

 have been an abundance of water, it is not at all improbable that many 

 of the species of Lymnseas may have retreated to this area and thus 

 survived to aid in the repopulation of the north after the recession of 

 the ice sheet. 



Before considering the post-glacial dispersion of the Lymnseid 

 fauna it would seem necessary to review briefly the fauna previous 

 to the Glacial Period and note its derivation. It has been shown in 

 the chapter on Distribution in Time that up to the Pliocene Period 

 there existed a more or less varied Lymnseid fauna consisting of nearly 

 all of the present generic types. Some of these types may have arisen 

 independently, as they are found in both Europe, Asia and America ; 

 but a few of the types, as, for example, the typical Lymnseas repre- 

 sented by stagnalis, undoubtedly reached America by way of Asia, 

 before the final separation of the two continents by Bering Strait. 

 It will thus be seen that previous to the extended englaciation the 

 Lymnseid type was quite fully developed, including, probably, a large 

 number of species. It is also probable that the preglacial fauna was 

 about as it is today, if we may judge from the few inter-glacial and 

 post-glacial fossils obtained and from fossil remains found beyond the 

 area of the drift. 



