250 0. C. Marsh — Discovery of Cretaceous Mammalia. 



In the present article, as in the two preceding it, the remains 

 selected as types are all characteristic specimens, which, 

 although fragmentary, will admit of accurate determination 

 whenever more complete material is available for comparison. 

 This is a point of much importance, as paleontology has 

 suffered grievously from descriptions of fossils without charac- 

 ters sufficiently definite to distinguish them from more perfect 

 remains subsequently brought to light. It is a matter of much 

 less importance if such discoveries should prove that two or 

 more specimens described as distinct really pertained to one 

 animal. The advance of the science throughout the world 

 has not been retarded by such preliminary reference, but has 

 often been greatly promoted by making known promptly 

 single facts of importance, leaving their full significance to be 

 determined by later discoveries, made under more favorable 

 circumstances. 



The fact, that, for half a century or more, search for 

 mammals in the Cretaceous had been made in vain, indicates 

 the importance justly attached to their recent discovery, and is 

 a sufficient excuse, if any were wanting, for making known 

 the little now accomplished by continued and laborious explora- 

 tions. The diminutive size of the Cretaceous mammals found, 

 and the nature of the deposits in which they were entombed, 

 readily account for the incomplete and isolated fragments thus 

 far secured. The determination of such remains must of 

 necessity be more or less provisional. It will be seen, however, 

 from the specimens figured and described in the present 

 article, that among the great number obtained, there are some 

 which give valuable information in regard to mammalian life 

 in Cretaceous time. 



The geological lesson now taught by these mammalian relics 

 and their associated vertebrate fossils is no less important, but 

 hardly what was expected. These remains are not transitional 

 between Mesozoic and Tertiary forms, but their affinities are 

 with the former beyond a doubt ; thus indicating a great 

 faunal break between the time in the Cretaceous when they 

 lived and the earliest known Tertiary, or between the Ceratops 

 horizon and the Coryphodon beds of the Eocene Wahsatch. 

 The lower division of the Coryphodon beds, or lower Wahsatch 

 (Puerco), is clearly Tertiary, and the great break is between 

 this horizon and the Ceratops beds of the Laramie. Each of 

 these faunas is now known by many species of vertebrate 

 fossils represented by hundreds of specimens, and the more 

 the two are compared the stronger becomes the contrast 

 between them. Instead of placing them close together, as 

 some geologists seem inclined to do, it will be more profitable 



