OF NORTH AMERICA. 53 



taken to represent approximately the outline and extent of the princi- 

 pal portions of the North American continent that were above the 

 level of the sea at the beginning of the Mesozoic time. A broad ex- 

 panse of Mesozoic sea then stretched between these two continental 

 factors, which were finally united by a general continental elevation 

 and the consequent recedence of the sea. This elevation was not, 

 properly speaking, catastrophal, but gradual and oscillatory." 



Just how this dispersal was brought about, it might, perhaps, be 

 difficult to state with certainty. It may have been by means of drift- 

 wood, freshets or by some of the birds or mammals. Much of the 

 widening of the range was doubtless accomplished by the ordinary and 

 natural means of locomotion. As the continent was gradually lifted 

 above the sea, the interior basin became a large inland sea, then a 

 brackish lake and was finally broken up into the large salt lakes now 

 found in the western United States, the larger part of the water 

 draining off or evaporating as the land rose higher and higher. The 

 many rivers and streams which drain the country have formed ready 

 avenues for the almost universal dispersal of the Limnaeid fauna. 



While it is probable, if not certain, that the American Lymnaeid 

 fauna originated in the fresh waters of this ancient land, it is also 

 true that there was an admixture of Asiatic types of Lymnaea, which 

 reached this region during Mesozoic and Cenozoic time via the land 

 connection between the two continents at Bering Sea. That the Rocky 

 Mountains, which have proven such an effectual barrier to the east- 

 ward migration of the Asiatic Helicidse 1 and Unionidae 2 were not so 

 effectual in checking the later dispersal of the Lymnaeas is evidenced 

 by the presence of such species as Galba binneyi, Galba apicina, Galba 

 palustris, Galba obrussa, Galba humilis modicella and Lymnaa stag- 

 nalis appressa, on both sides of the Rocky Mountains. The discovery 

 of a large fossil Lymnaea (L. stearnsi) of the staghalis type in the 

 Middle Miocene beds of Oregon seems to indicate that at least some 

 of the Lymnaeas accompanied the Asiatic Helicidse and Unionidae in 

 their Mesozoic migration along the Pacific Coast. A number of spe- 

 cies, such as Lymncea lepida, Polyrhytis utahensis, Galba gabbi, Galba 

 bulimoides, Galba ferruginea, Galba proxima, Galba traskii, and Galba 

 sumassi, have, however, failed to cross the continental divide, but 

 these are not typically Asiatic, as is the case with the Unionidae and 

 Helicidae mentioned, but have apparently descended from some Ameri- 

 can ancestors. The truly Asiatic species, such as palustris and stag- 



, Guide to Study of Helices, p. XLHI, 1894. 

 2 Walker, Proc. Mai. Soc., IX, p. 129, 1910. 



