84 THE CHICAGO ACADEMY OF SCIENCES. 
It would appear that since early Eocene Time this plateau region, 
known later as the Great Basin, had been the scene of some of the 
most stupendous physical changes in the history of the earth. At the 
close of the Mesozoic Era this region was notably subjected to warp- 
ings and faultings which elevated much of the land, producing irregular 
mountainous tracts capable of rapid erosion. Basins were formed here 
and there in which lakes of large size developed.’ It seems not unlikely 
that from this time (the Lower Eocene Period) to the present there 
existed in the area of the Great Basin a continuous series of lakes, 
large and small, of which the Pleistocene lakes Lahontan and Bonne- 
ville were the last, and of which Great Salt Lake is a diminishing 
descendant. . 
That the fresh-water faunas contemporaneous with the Lymnezas 
were relatively large and varied in character is attested by the presence 
ei tire genera Unio, Auricula, Tortacella, Planorbis, Physa, Neritina, 
Pachymelania, Pyrgulifera, Hydrobia, Vivipara and Campeloma. Nu- 
merous forms of land shells also occur.? 
It also seems possible, if not probable, that the present Lymneeid 
fauna of this region is a direct descendant from these early times. It 
seems not at all improbable that the smaller Lymnzeas of the United 
States (techella, desidiosa, etc.) originated here, as well, also, as some 
of the larger species. That the recent Lymnea utahensis of Call is a 
lineal descendant of Meek’s Polyrhytis kingi seems evident. Lymnea 
stearnsi is also an early form of Lymnea stagnalis. 
It also seems apparent that the Lymnzas of American origin 
spread from this region, following the river systems as they were 
formed, and finally mingled with the Asiatic emigrants which reached 
the plain area by the way of the lowlands of Alaska and British 
America. This combined fauna worked its way to the higher regions 
of the Rocky Mountains, where it is found today intermingled with 
the remnants of the original American stock. It is not believed by 
the writer that the supposed land connection with Europe via Green- 
land contributed to any extent in the formation of the present Lymneid 
fauna, 
In the following pages an attempt has been made to bring together 
all references to extinct American Lymnzas, together with the original 
descriptions and figures. No attempt has been made to critically revise 
the species, but remarks are introduced wherever an opportunity has 
iGhamberlin and Salisbury, op. cit., III, p. 204, etc. 
2See the interesting paper by Dr. Stearns on the Colorado Desert, which 
doubtless once formed a part of the Great Basin system, Proc. U, 8. Nat. Mus., 
XXIV, pp. 271-299. 
