LYMNIDZ 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 Tange 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 Limnzid fauna, 
While it is probable, if not certain, that the American Lymnezid 
fauna originated in the fresh waters of this ancient land, it is also 
true that there was an admixture of Asiatic types of Lymnea, 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 Helicidee and Unionide? were not so 
effectual in checking the later dispersal of the Lymnezeas is evidenced 
by the presence of such species as Galba binneyi, Galba apicina, Galba 
palustris, Galba obrussa, Galba humilis modicella and Lymnea stag- 
nalis appressa, on both sides of the Rocky Mountains, The discovery 
of a large fossil Lymnza (L. stearnsi) of the stagnalis type in the 
Middle Miocene beds of Oregon seems to indicate that at least some 
of the Lymnzas accompanied the Asiatic Helicidze and Unionide in 
their Mesozoic migration along the Pacific Coast. A number of spe- 
cies, such as Lymnea lepida, Polyrhytis utahensis, Galba gabbi, Galba 
bulimoides, Galba ferruginea, Galba proxima, Galba trasku, and Galba 
sumassi, have, however, failed to cross the continental divide, but 
these are not typically Asiatic, as is the case with the Unionidz and 
Helicide mentioned, but have apparently descended from some Ameri- 
‘Can ancestors. The truly Asiatic species, such as palustris and stag- 

1Pilsbry, Guide to Study of Helices, p. XLIII, 1894, 
?Walker, Proc. Mal. Soc., IX, p. 129, 1910. 
