4 AMERICAN MESOZOIC MAMMALIA 



After Marsh's retirement from the field, further collecting in the Jurassic has 

 been largely negative in results, but knowledge of Cretaceous mammals of the world 

 has been continually increasing to the present time. In 1892 Wortman discovered a 

 fragmentary specimen which had the peculiar virtue of showing upper and lower teeth 

 in association. This important form Cope described as Thlaeodon fadantcus. In the 

 same year Wortman and Peterson made a collection for the American Museum at 

 Marsh's Wyoming localities which was described in a paper by Osborn in 1893. In 

 treating the early evolution of the Amblypoda in 1898 Osborn returned to some of this 

 material and applied names to three types left nameless in the earlier work. 



Save for the rather doubtful Paronychodon, all the American Cretaceous mam- 

 mals so far known had come from the Lance and its equivalents, but in 1902 Lawrence 

 Lambe described two new forms from the Belly River of Alberta, a horizon equivalent 

 to the Judith River of Montana and older than the Lance. Working in the same beds 

 in 191 5, Barnum Brown discovered an unusually fine specimen, later described by 

 W. D. Matthew ( 1916) as Eodelfhis brozvni. Another nearly complete lower jaw from 

 the same formation was sent to the British Museum by William E. Cutler and was 

 named Cimolestes cutleri. by A. Smith Woodward in a paper appearing almost simul- 

 taneously with that of Matthew. 



In 191 5 R. S. Lull published a detailed and valuable stratigraphic paper on the 

 Lance mammal occurrences, but only a few small and more or less casual collections 

 have since been made. It remains only to mention that a small but important collection 

 had been made in 1907 in Montana beds equivalent to the Lance by Barnum Brown. 

 This collection was not described until 1927 (Simpson, 1927A). 



The importance of the Mesozoic mammals in the literature has been much en- 

 hanced by the central place which they have rightly come to occupy in the numerous 

 theories of molar evolution, especially in the arguments of what has been called the 

 American School. Osborn's Evolution of Mammalian Molar Teeth (1907) contains 

 the essence of his deductions in this respect, and also a new classification of Mesozoic 

 mammals which serves as a point of departure for later work. In 1906 J. W. Gidley 

 published excellent figures of a few of the teeth in the United States National Mu- 

 seum and based on them a criticism of the Cope-Osborn theory. 



Expressed as briefly as is consistent with proper recognition of a great debt to the 

 past, such are the steps by which the foundation for the present more general work was 

 laid. The scope of this memoir is adequately represented by its title — all American 

 Mesozoic mammals are included, with the understanding that for present purposes the 

 Lance is accepted as the last Mesozoic and the Puerco as the first Tertiary horizon. 

 The problems here considered are threefold : morphologic, taxonomic, and geologic. 

 The morphologic results are kept objective, as far as possible, and are documented 

 largely by illustrations of the critical material. The taxonomic results are based on this 

 newer and more extensive morphologic knowledge, on detailed comparisons with 

 European material, and on the general advance in knowledge of the last years. They 

 have a firmer basis in objective observation than has hitherto been possible, but like all 

 previous classifications this one is obviously liable to radical revision by future work. 



