LYMN@ID4 OF NORTH AMERICA. To 
doubtless afforded ample havens for the mollusks which were driven 
from the frozen territory.’ 
To adequately understand the rapidity with which the newly 
opened territory was occupied by the Lymnezas, it must be borne in 
mind that these mollusks are able to withstand a very low temperature. 
Many of the species today live in ice-cold water and may be collected 
in ponds and streams when the surface of the water is frozen over. 
They were thus enabled to follow the retreating ice very closely and 
so to quickly occupy the new territory as it became available. This 

EG) 4s 
Glacial Lake Iroquois showing Mohawk River outlet. (After Gilbert, U. S 
Geol. Surv.) 
is clearly indicated by the discovery of the remains of Lymneas, as 
well as of other boreal mollusks, in various inter-glacial deposits, as 
at Cayuga Lake’ and in the Don Valley at Toronto.? As there is 
believed to have been a period between each ice sheet, during which 
time there was a comparatively mild climate, it is to be supposed that 
the Lymnzas migrated into this territory, which they occupied at each 
successive inter-glacial period. 
TERTIARY LAKES OF THE GREAT BASIN. Figures 2 and 5. 
During the Glacial Period the western area known as the Great 
Basin again developed several large lakes, the largest known as Lake 
Bonneville, attaining a maximum depth of 1000 feet and an area of 

3See Chamberlain, Geol. of Wis., Vol. 1, plates 9 and 10. 
1Maury, Journ. Geol. XVI, p. 565, 1908. 
Coleman, Interglacial Periods in Canada, 1906. Also, Journal Geol. IX, p. 
285, 1901; Amer. Geol., XIII, p. 85, 1894. 
