54: THE CHICAGO ACADEMY OF SCIENCES. 



nails 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- 

 naeas, 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 Lymnseas of Greenland are related 

 to the American fauna, while those of Iceland are related to the Euro- 

 pean fauna. The species vahlii and vahlii pingclii 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. 



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 



