64 BIOLOGICAL SOCIETY OF WASHINGTON. 



forms, differing so much from those of other parts of the world that 

 they are designated among, naturalists as " North American types." 

 The Uniones of the Laramie group, of which a considerable number 

 of species have been discovered, are mainly of these North Ameri- . 

 can types, and I, therefore, conclude that these fossil forms represent 

 the living ones ancestrally. This conclusion of course implies that 

 there has been an unbroken habitat for those fresh-water mollusks, 

 from the Laramie period to the present time. Accepting this con- 

 clusion, we necessarily understand that the outlet of the Laramie 

 sea continued to flow as a river after the disappearance of that in- 

 land sea and down to the present time, and that that river is now 

 the Missouri, or one of its tributaries. The Laramie deposit also 

 contains the remains of certain ganoid fishes, which are closely re- 

 latedto the gars i^Lepidosteus) and dog-fishes {Amia) of the Mississippi, 

 and the fishes doubtless effected their descent in the same waters 

 with the Uniones. 



If geologists have read the later history of the North American 

 continent correctly, we learn that at the time those ancient inland 

 bodies of water existed, the great Southern Gulf extended so far to 

 the northward that it probably received the outlets of those bodies 

 of water as separate streams. The same conditions would also have 

 made the Ohio and Upper Mississippi separate rivers, emptying by 

 separate mouths. While these two last-named rivers were separate 

 from the western one, which drained the lakes and the inland sea, 

 they doubtless had faunas which were quite different from that 

 which now characterizes them. When, by a recedence of the 

 borders of the gulf to the southward, all those rivers united their 

 waters to form the main stream of the Mississippi, it is easy to see 

 how the ancient fauna, which had come down the Missouri branch, 

 may have become dispersed throughout the great river system. 



I have thus endeavored to point out from among the great mass of 

 evidence of the existence bf life upon the earth during successive 

 geological periods what portions of that evidence have reference 

 to then existing land areas, for the bulk of it tells us of the ever 



