LYMN^EID.E OF NORTH AMERICA. 55 



States to the west, absolutely destroyed all life north of the above 

 limits. As the ice sheet receded, the remnants of the Lymnseid fauna 

 again advanced and took possession of the newly formed lakes and 

 rivers. That the preglacial Lymnaeid fauna was nearly identical with 

 that of the present fauna we believe from evidence afforded by the 

 identity of fossils of interglacial 1 and post-glacial deposits with the 

 existing fauna. The question of the means by which the present Lym- 

 naeid fauna has become so widely distributed need not enter largely, 

 however, into the present discussion, since its purpose is mainly to 

 consider the present distribution of North American Lymnaeas, draw- 

 ing from this study any useful of interesting inferences which may 

 present themselves. 



The methods by which the Lymnaeas have become so widely dis- 

 tributed are probably many and diverse. We know that they are car- 

 ried by floods for great distances, attached to drift wood and to other 

 objects, and the boats which ply up and down our rivers have doubt- 

 less helped to disperse these mollusks. The Erie Canal, in New York, 

 is known to have been the highway for the dispersal of many mollusks. 

 The veteran conchologist, Dr. Lewis, has stated on several occasions 

 his belief that the transfer of species of Lymnaea from the canal to 

 the rivers of New York state has produced some of the species of this 

 genus (c. f. elodes and catasc opium). While the author cannot agree 

 with this theory, he still knows that Lymnaeas and other fresh-water 

 mollusks have been carried from one part of the canal to another by 

 means of the canal boat, on several occasions mollusks having been ob- 

 served clinging to the sides of these freighters. Water birds, es- 

 pecially ducks, are known to carry mollusks from one place to another, 

 attached to their feet or some other part of the body; it is even re- 

 corded 2 that the eggs of Radix auricularia have passed through the 

 digestive system of a swan without injury. 



Various zoologists have divided the North American continent 

 into zoogeographic regions, based on the distribution of the different 

 species in latitude. Some early writers designated the country north 

 of Mexico as the Nearctic region. Other authors have given the name 

 Holarctic to the region including North America, Europe, northern 

 Asia and Africa. Still others divide it into Arctic, Nearctic and Neo- 

 tropical (see H. Jordan, 1883). While it is true that the fauna of this 

 region is more or less uniform, it is also true that there are reasons 



1 See Coleman for a discussion of these deposits. Lymnaeas recently dis- 

 covered in deposits formed during the early stages of glacial lake Chicago are 

 for the most part identical with recent species. (Science n. s., XXXI, p. 715.) 



2 Pascal, Journ. de Conch., XXXI, p. 9, 1891. 



