1896.] 1 4 J [Farr. 



Moreover the description of this species de novo is justifiable because we 

 wish to trace the steps iu the evolutioa of the horse as they can be fol- 

 lowed in the horizons of the White River strata and must therefore have 

 a description of one species as a standard for comparison. 



It is the purpose of this paper to add some new points on the osteology 

 of M. baircU and to give a new, more accurate and more complete restora- 

 tion ; to give a short description of 3/. intermcdius and M. cupei, and to 

 show their relation to each other and to M. hairdi. 



I must acknowledge my very great indebtedness to Prof. Scott, who 

 has given me so much assistance in the way of suggestion and criticism 

 and whose kindly interest in my work has ever been an inspiration dur- 

 ing my three years of graduate study in Princeton. To Mr. J. B. 

 Hatcher I am also very much indebted for free access to collections and 

 for kindl}' criticism and help and for much information on White River 

 mammals. 



I must also extend my thanks to Prof. H. F. Osborn and Dr. J. L. 

 Wortman, of the American Museum, for permission to study some of 

 their very beautiful material ; also to the latter for valuable suggestions. 



The drawings are by Mr. R. Weber, and add materially to the value of 

 the paper. 



The Dentition 



The dental formula is I. |, C. \, Pm. f, M. f. The dentition is thus 

 seen to be unreduced, and the specialization or modernization consists in 

 the complexity of the last three premolars which are molariform and 

 Pm. 2 is beginning to assume the elongate character, so marked in the 

 living horse by the elongation of the anterior part of the external half 

 of the tooth. The characters of the permanent teeth have already been 

 described by Leidy,* Osborn, f and Scott,:}: but very little has been 

 written concerning the milk dentition and the superior incisors have 

 only very recently been found. Only two skulls are known bearing the 

 upper incisors, nearly all the skulls that are discovered having the end 

 of the very narrow snout broken oft". 



The inferior canine is the smallest of all the teeth ; it is suberect and 

 conical, and there is a wide diastema between it and Pm. 1. The lower 

 incisors are spatulate or chisel shaped and do not show any indication of 

 a depression or pit. Thej' have sharp cutting edges, and their inner sur- 

 faces are strongly concave. The first incisor is the longest {i. e., highest 

 above alveolar border) and also the widest of the incisor series. I. 2 is 

 smaller than I. 1, while I. 3 is the smallest of the incisor series. There 

 is thus a decrease in size and length of incisors outwardly towards the 

 canine. The six incisors form an unbroken row. 



* Ancient Fauna of Ntbmska, pp. 70, 71 ; Extinct Mam. Fauna of Dak. and Neb., pp. 3r5- 

 309, 1869. 

 t Bnl!. of Mus. of Covip. Zoij!., Vol. xvi, pp. 88, 89. 

 f'Osrteologyof Mesohippusand Leptomeryx," Journ. of Morph., Vol. v. No. 3, pp. 303-305. 



