468 THE MAMMALIA OF THE UINTA FOllMATION. 



abundant in the Uinta, is the rhinoceros-like Amynodon, a hornless form which is fol- 

 lowed in the White River by Metamynodon and Aceratherium. The TTinta tapir is 

 Iscotolo'phus, which is as yet represented by a single abundant species of small size. 

 This genus appeals to be represented also in the Bridger, at least there is a well-pre- 

 served specimen in the Princeton Museum which we cannot distinguish from it, and 

 seems to have its representative in the White River beds, which Dr. Leidy has 

 doubtfully referred to Lojiliiodon, though only for the reason that the specimen, an 

 isolated last lower molar, could not well be distinguished from the corresponding 

 tooth of that genus. As Leidy has suggested, this specimen most probably belongs 

 to a very different genus (No. 7, p. 239). The equine series is represented in the 

 Uinta fauna by two species of Orotherium, (JSjnhijjpus) which stands just interme- 

 diate between PUoloj^Ims ( Oroldpims) of the Bridger and Mesoliip)pus of the White 

 River, though it also is found in the Bridger. 



Perhaps the most striking change in the facies of the Uinta fauna as compared 

 with that of the Bridger lies in the importance suddenly assumed, by the selenodont 

 artiodactyls. The Bridger is not known to contain any forms which can be strictly 

 called selenodonts, though Homacodon and Helolryus are obviously commencing to 

 develop this type of dentition. The Uinta contains at least two clearly mai-ked gen- 

 era of selenodonts, if not more, which are very rich in individuals and thus give a 

 very diflei-ent aspect to the fauna as a whole. Of these the most abundant is Pro- 

 toreodo7i, the unmistakable ancestor of the Oreodontidce, the peculiar family which is 

 so eminently charactei'istic of the American Miocene. By the five-lobed construction 

 of the upper true molars, Protoreodon indicates the line of connection with the buno- 

 selenodonts of the earlier Eocene, especially, if we may judge from the molar teeth 

 alone, with Ilelohyus. The other clearly defined Uinta selenodont is Leptotragulus, 

 the most ancient unequivocal member of the Tylopoda and closely related to the 

 White River genus Poehroiherium, though in many respects more primitive. The 

 absence of the upper molars from our specimens prevents comparison of this type 

 with Homacodon, from which it was perhaps derived. The Uinta has thus yielded 

 forerunners of the two most important and characteristic groups of White River sel- 

 enodonts ; it remains for future investigation to bring to light the ancestors of Lepto- 

 riieryx and Hyopotarims, though the latter is not improbably an immigrant fi'om the 

 Old Woi'ld. No Bunodonta have as yet been found, and the origin of the White 

 River genera, Perchocrus, Entelodon, etc., is still obscure. Our knowledge of the 

 Bridger bunodonts is still very imi^erfect. 



It will be evident from the above statements that the aillnities of the Uinta 

 fauna are most closely with the Bridger, as at least five genera are common to the 



