624 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



able to assume that much of the differentiation that now prevails among 

 the living ]S"orth American Unionidce took place in purely fresh, and 

 especially in fluviatile waters, the facts brought out by the study of the 

 fossil forms seem to indicate plainly that the characteristics which we 

 call " North American " have been directly inherited from those fossil 

 species, and the probability also that the latter species received in 

 Mesozoic and Tertiary times their differentiation under the influence of 

 other conditions, among which was the diffusion of a small proportion 

 of salt in the waters in which they lived. Before this i)roposition can 

 be received without question, however, much careful investigation must 

 be made; and it is especially desirable that numerous observations be 

 made upon the Unionidce of various rivers at the meeting of fresh with 

 salt waters. 



If the progenital line of the North American Unionidce has remained 

 unbroken from the earlier Tertiary epoch to the present time, as is be- 

 lieved to have been the case, it was doubtless accomplished through 

 some streams that are now western tributaries of the great Mississippi 

 Eiver system, and which were then outlets of those great lakes in the 

 deposits of which the fossil Unionidce are now found. It is comparatively 

 easy to understand how this might have been the case with the lakes 

 that formerly existed in what is now the Upper Missouri River region, 

 because that river now runs through the region and has doubttbss done 

 so ever since the final desiccation of the great lakes there. The Green 

 Eiver region, however, which was the site of probably the largest fresh- 

 water lake that ever existed on the North American continent, is now 

 drained into the Gulf of California; but it is not at all improbable that, 

 during its existence, this great lake had its drainage of overflow into the 

 Atlantic by some stream that is now either a direct or indirect tributary 

 of the Mississippi River, and that this was the channel through which 

 the Molluscan fauna of that ancient lake was transmitted by lineal de- 

 scent to the waters of the great existing river-system last named. It is 

 not difflcult to understand bow the ordinary drainage of the region, after 

 the desiccation of the lakes which had occupied so large a part of it, 

 might have become turned toward the Pacific instead of the Atlantic 

 Ocean, by the continuation of those varying changes of elevation of dif- 

 ferent portions of the continent which caused the desiccation of the 

 lakes, and produced or modified other great features of the gradually 

 growing continent. Indeed, the fact that the Molluscan fauna of the 

 great Colorado River system is very meager, and unlike that of the great 

 fresh-water deposits of the region which it drains, and that it contains 

 none of those types so often referred to as distinctively North American, 

 while the great Mississippi River system constitutes the great habitat 

 of those types now living, seems to afford presumptive proof that the 

 drainage of overflow from the great lakes of what is now the Green 

 River region was into streams that have since become tributaries of 

 the Mississippi. 



