LYMNID& 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 Lymnzid fauna 
again advanced. and took possession of the newly formed lakes and 
rivers. That the preglacial Lymnzid fauna was nearly identical with 
that of the present fauna we believe from evidence afforded by the 
identity of fossils of interglacial’ and post-glacial deposits with the 
existing fauna. The question of the means by which the present Lym- 
nid 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 Lymnzas, draw- 
ing from this study any useful or interesting inferences which may 
present themselves. 
The methods by which the Lymnzeas 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 Lymnza from the canal to 
the rivers of New York state has produced some of the species of this 
genus (c. f. elodes and catascopium). While the author cannot agree 
with this theory, he still knows that Lymnzeas 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’ 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 
4See Coleman for a discussion of these deposits. Lymnzeas 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, pe 715.) 
*Pascal, Journ, de Conch., XXXIT, p. 9, 1891. 
