454 II. F. Osborn — Mammalia in North America. 



Stonesfield slate ; there is a relatively shorter interval, but 

 still a considerable one between this and the Purbeck or Atlan- 

 tosaurns beds. Then follows another long and very important 

 interval between the Atlantosaurus beds and the Laramie 

 (Upper Cretaceous). The gap between the Laramie and 

 Puerco was relatively short as indicated by the comparatively 

 limited evolution both of the Plagiaulacids and Trituberculates. 

 The Puerco itself was a long period in which the Plagiaulacids 

 underwent considerable changes. Then follows an interval 

 which it is most important to fill by future exploration, for 

 between the Puerco and the Wahsatch the differentiation of 

 the even and the odd- toed ungulates must have occurred. The 

 Wahsatch proper does not mark a very extensive evolution of 

 the forms it contains. It passes after a slight break into the 

 base of the Bridger (Wind River) and then begins that splen- 

 did and almost uninterrupted succession of lake basins, termi- 

 nating in the Pliocene. I append a table, to be compared with 

 that published by Marsh in his admirable address of 1877, and 

 exhibit the great progress of the last sixteen years. 



The general faunal succession is marked by the sudden 

 appearance and disappearance of certain series and rise and 

 fall of great groups. In the Trias appears the remarkable pro- 

 todont or primitive-toothed Dromatherium ; we cannot deter- 

 mine its Order at present, We still have no American fauna 

 corresponding to the intermediate Stonesfield of England. In 

 the Jurassic Atlantosaurus beds the three supposed representa- 

 tives of the Monotremes (multituberculates), Marsupials (trico- 

 nodonts) and Placentals (trituberculates), appear in equal num- 

 bers ; the latter are generally characterized by the primitive 

 dental formula. In the Laramie the Multituberculates con- 

 tinue in great profusion, and the Marsupials and Placentals are 

 also numerous. 



The serial succession of the Trituberculates from the Meso- 

 zoic is still an unknown chapter ; we are utterly unable to con- 

 nect the Dromatheriidse of the Trias, the Triconodontidse, 

 Amphitheriidse and Amblotheriidse of the Jura with each 

 other, or with any Cretaceous or lower tertiary mammals. The 

 serial relations of the Multituberculates, on the other hand, 

 have been made much clearer by the discovery of the Laramie 

 fauna. Cope and Marsh in this country, and Smith Woodward 

 in England, have at last broken into the long barren Cretaceous. 

 In studying the accurate figures published by Marsh and a 

 large collection of teeth recently made for the American 

 Museum by Wortman and Peterson, I find that this Laramie 

 fauna is widely separated from the Jurassic in its general evo- 

 lution, and as Gaudry, Lemoine and Cope have observed, it 

 approaches more nearly the basal Eocene of the Puerco and 



