54 THE CHICAGO ACADEMY OF SCIENCES, 
nalis have broken over every barrier and are widely distributed over 
the continent. 
The northeastern Greenland-Iceland connection, so ably advocated 
by Dr. Scharff, does not appear to have been made use of by the Lym- 
nzeas, the characteristic European species stagnalis and palustris being 
absent from Greenland and northeastern America. A fact that bears 
strongly against the use of this land connection is the discovery, made 
by Morch many years ago, that the Lymnzas of Greenland are related 
to the American fauna, while those of Iceland are related to the Euro- 
pean fauna. The species vahlii and vahlu pingelii are found from 
Greenland to Alaska, which distribution seems to prove an Asiatic 
rather than an European origin. 
The following table of comparative distribution seems to support 
the Asiatic origin of the circumboreal forms. The absence of any 
indication of the common species stagnalis or palustris in either Green- 
land or Iceland, especially as the former is not believed to have been 
rendered more uninhabitable along the coast than at the present time 
(in which case there should have been survivors of the fauna) is 
ample evidence that the fresh-water pulmonates migrated eastward 
across Europe and Siberia and entered America by way of the Bering 
Sea land connection, which has been available many times since Lower 
Cretaceous (Comanchean) time. 

Eastern N. W. Greenland and 
Siberia. America. N. E. America. Iceland. Europe. 
L. stagnalis stagnalis Ss __ stagnalis 
L. peregra a SSS peregra peregra 
L. auricularia.=£————— —s Ss °° atricularia 
L. palustris palustris 
——_—_—_— vahlii vahlii ——_—_—— palustris 
L. truncatula  truncatula holbollii truncatula truncatula 
Pl. albus albus arcticus rotundatus albus 
A. hypnorum  hypnorum hypnorum 
There have evidently been several Asiatic invasions in which some 
of the species accompanied the Helices and Uniones down the Pacific 
Coast, while other species, possibly at a later time, when there was 
a land connection between the two continental masses, migrated toward 
the southeast and into the central plains area. 
The great Glacial Epoch, which occurred near the close of the 
Tertiary Period, has greatly influenced the present distribution of the 
fresh-water pulmonate Mollusca. The vast sheet of ice which covered 
the North American continent as far south as southern Illinois, In- 
diana and Ohio to the east and the extreme northern part of the United 
