INTRODUCTION 3 



forms, and the avowedly incomplete nature of Professor Marsh's studies made a thor- 

 ough synthesis impossible. The fact that this memoir and Marsh's paper of 1887 have 

 remained the authorities for Jurassic mammals for over forty years is a monument to 

 their high quality. That many changes, some fundamental, are now necessary bespeaks 

 only the inevitable advance of knowledge, the availability of materials then unstudied, 

 and the possibility of first-hand comparisons then impossible. 



Although the earlier Mesozoic mammals were thus slowly being made known, 

 both in Europe and in America, there remained a vast gap between the close of the 

 Jurassic and the dawn of the Tertiary in which no mammals were known. The filling 

 of this gap has been very slow, and it remains in essence the greatest single break in 

 our knowledge of mammalian history. A number of vitally important Cretaceous 

 mammalian faunas have, however, since come to light. 



In 1876 Cope described a single tooth from the upper Cretaceous of Montana 

 under the name Paronychodon, a genus now recognized as mammalian although de- 

 scribed as reptilian. In 1882 J. L. Wortman found in beds of similar age in South 

 Dakota two teeth and a fragmentary humerus which Cope named Menisco'essus con- 

 quistus, the first recognized Cretaceous mammal. One of the teeth later proved to be- 

 long to an armored dinosaur, but this cannot be supposed to invalidate the discovery. 



The whole problem of mammals in the upper Cretaceous remained rather dubious, 

 however, until 1889 when J. B. Hatcher was in what was then Converse County' in 

 Wyoming, collecting dinosaurs. A futile search had been made in Montana and South 

 Dakota for mammals during the preceding year, but in the Wyoming field Hatcher 

 was amply compensated for previous disappointments by the discovery of quantities of 

 mammal teeth near his ceratopsian dinosaur localities. Between 1889 and 1892 were 

 made the large collections, comprising many hundreds of isolated teeth, fragmentary 

 jaws, and skeletal parts now preserved at Yale and in the National Museum. Hatcher 

 was assisted by several other workers, of whom O. A. Peterson is especially worthy of 

 mention, and in 1889-90 Professor Beecher of Yale was also in the field and made 

 large collections. These tiny and fragmentary remains were almost all picked up on 

 the surface where they had accumulated as the finer material was blown away by the 

 wind. The majority of them were found in the numerous ant hills of the region, a 

 unique occurrence of fossils — collected by insects in their underground burrows, 

 brought to the surface with other material, sifted by the wind, and awaiting only the 

 selective agency of man. (See Lull, 191 5a, -b.) The results of these efforts were em- 

 braced in a series of three papers by Marsh (1889A, -b, 1892). Here again his work 

 was intended only to be preliminary and a synthesis was left for a projected mono- 

 graph which he did not live to complete. 



In 1 89 1 parts one and two of Marsh's three papers, collectively titled "Discovery 

 of Cretaceous Mammalia," were critically reviewed by Osborn, who attempted the first 

 correlation of the tooth types into natural genera. Marsh's few further publications on 

 Mesozoic mammals were chiefly of a polemic nature and his monumental labors on this 

 subject must be considered as virtually ending in 1892. 



' This old county has since been divided and the locality in question is now in Niobrara County. 



