No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 303 



have broad crowns with distinct pits or invaginations of the 

 enamel, while in the latter the crowns are sharp and chisel- 

 shaped, very much as in Hyracotherium, without any trace of 

 such invagination. (Cf. Cope, No. 9, PI. XLIX. a, Fig. 3 ; and 

 Kowalevsky, No. 25, PI. III., Fig. 57.) 



There are, besides, ^not unimportant differences in the con- 

 struction of the molars and of various parts of the skeleton, 

 which show that Mesohippus is a more primitive form than 

 Anchitherium, and indeed it may be doubted whether the Euro- 

 pean type is in the line of equine descent at all, but does not 

 rather form a side-branch. 



It is not necessary for our purpose to enter into the question 

 whether the equines of the John Day Miocene should be referred 

 to AncJiitJieriam or not. 



Though the White River species has so long been known, our 

 knowledge has been chiefly confined to the structure of the skull 

 and the dentition, which have been described and figured by 

 Leidy. Cope has published some valuable notes on the skeleton 

 (No. n), as has also Marsh (No. 35), but these are too brief for 

 the object of this paper. Kowalevsky's excellent memoir upon 

 Anchitherium (No. 25) was never completed, and is to some 

 extent vitiated by the supposition, now abandoned, that Palceo- 

 therium is the primitive horse-type. 



I. The Dentition. 



The dentition has been so fully described by Leidy (No. 33), 

 Osborn (No. 42, p. 88), and others, that a brief summary is all 

 that is necessary here. 



1. Upper Jaw. The superior incisors and canines are not pre- 

 served in any of the specimens which I have seen, but the alveoli 

 for them show that they were present in unreduced number, and 

 that they were of small size. The first premolar is a small, com- 

 pressed, and simple cone, with a rather strong cingulum devel- 

 oped on the inner side of the crown. The other premolars are 

 almost exactly like the molars in construction, with very slight 

 differences of detail ; thus the posterior pillar is less developed 

 and sometimes altogether absent ; the posterior transverse crest 

 is more completely divided from the postero-internal cusp. In 

 other respects the transformation is complete. The second pre- 



