10 SCOTT. [Vol. V. 



this form is very imperfectly known and is of no importance for 

 our purpose. The Uinta representative of the series is Leptotra- 

 guilts, Scott and Osborn ; Parameryx, Marsh, of the same forma- 

 tion, is a mere nomen nudum, as to which we have no information. 

 In the White River beds is found Pcebrotherium, which also 

 occurs in the John Day, and is followed by Protolabis, Cope, of 

 the Deep River and Loup Fork, Procamelus, Leidy, Homocame- 

 lus, Leidy, Protomcryx, Leidy, and Megalomeryx, Leidy, and 

 Pliauchenia, Cope, of the Loup Fork. The Pliocene yields the 

 genera Holomeniscus and Eschatius of Cope and ProtaucJienia, 

 Branco, the line terminating in Auchenia and Camelus of the 

 existing fauna. 



The Princeton expedition of 1890 had the good fortune to ob- 

 tain an almost complete skeleton of PcebrotJicrium of the lower 

 or White River Miocene, the middle member of the Tylopodan 

 series. This beautiful specimen, which was discovered by Pro- 

 fessor W. F. Magie, with other specimens in the Princeton mu- 

 seum, and some which have been very kindly put at my disposal 

 by Professors Cope and Agassiz, forms the main subject of this 

 paper. The plan proposed is to give a detailed description of 

 PcebrotJicrium, followed by a discussion of the evolution of the 

 skeletal and dental structures of the Camelidce, the phylogeny of 

 the family and its relation to other artiodactyls. In the second 

 paper a similar plan will be followed for other mammalian series, 

 and then an attempt will be made to apply the results of these 

 studies to the solution of the evolutionary problems already 

 enumerated. 



Pcebrotherium, Leidy. 



This genus is quite abundant in the lower and middle Miocene 

 formations of the West (White River and John Day). Hitherto 

 it has not been adequately described. Leidy's type specimen 

 is the much-fractured skull of an immature animal, which does 

 not allow a complete account to be given. Cope has published 

 some very valuable notes (Nos. 6, 8, 9, etc.) upon the vertebrae, 

 limbs, and teeth ; but these are all too brief to serve our pur- 

 pose, which requires a careful consideration of all the minute 

 points of structure. 



