62 BIOLOGICAL SOCIETY OF WASHINGTON. 



in this case brought into immediate juxtaposition. The great dino- 

 saurian fauna of the Laramie period doubtless originated much 

 earlier, and came down under continuously favorable conditions 

 from the earlier Mesozoic periods, and which favorable conditions 

 were not interrupted by the land movements by which the Laramie 

 sea was inclosed. Their final extinction appears to have resulted 

 from an unequal struggle for existence, which ensued upon the intro- 

 duction upon the land they had previously occupied of the immense 

 mammalian horde whose remains are found in strata immediately 

 overlying those which contain their own. The mammals thus be- 

 came the leading forms of life upon this continent, as it then 

 existed, as the dinosaurs had been during Mesozoic time. 



For reasons already stated, we assume that the progenitors of this 

 great mammalian fauna, whose appearance upon the earth seems to 

 have been so sudden, really existed somewhere long previous to the 

 time which is represented by the strata in which the remains referred 

 to are found. I regard it as probable that those progenitors occu- 

 pied some continental area, adjacent to that which the present one 

 now occupies, and which, after the migration of the fatma to the 

 region where their remains are now found, superimposed upon those 

 of the dinosaurs, became destroyed or submerged. 



It does not seem to me probable that this mammalian fauna could 

 have previously occupied either the eastern or western portion of 

 North America, as they are understood to have existed then, because 

 those portions were already united at the beginning of the Laramie 

 period, when the Laramie sea was formed, and their earliest known 

 remains are found in strata which were deposited after the close of 

 that period ; that is, I hold that the known facts warrant the opin- 

 ion that the immediate predecessors of the Tertiary mammalia re- 

 ferred to, themselves of Tertiary types, originated and long existed 

 contemporaneously with the dinosaurian fauna of the Laramie 

 period, but in a land which was separate from that which the latter 

 occupied. Furthermore, that the land area which the mammalia 

 then occupied is now unknown ; but that it became united to the 



