LYMNAIDZ 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 Lymneid 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 Lymnzas 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 Lymnzas 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 Lymnezid 
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 Lymnezid 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 Lymnzas 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 
Lymnezid 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. 
