No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 3 1 3 



the side of the atlas. The characters of the atlas of Mesohippus 

 are thus obviously equine. 



The atlas of AncJiitJiermm figured by Kowalevsky (No. 25, 

 PL I., Fig. 40), so far as it is preserved, closely resembles that 

 of Mesohippus. The notch which separates the anterior cotyli 

 above is not so broad or deep; the posterior cotyli face more 

 directly backward, and the foramina for the dorsal branches of 

 the first pair of spinal nerves are continued forward as deep 

 grooves. In other respects the two are alike. 



In the modern Equid<z the atlas has become broader and more 

 rugose and massive in construction ; the transverse processes 

 are extended forward, converting the notches at their bases into 

 foramina, and the posterior openings of the vertebrarterial 

 canals are considerably enlarged. 



The axis (PI. XXII., Figs. 5-9) of Mesohippus presents many 

 more differences from the modern type than does the atlas, and 

 is also subject to an unusual degree of individual variation. The 

 centrum is long, very much depressed and quite broad in front, 

 becoming narrower and deeper behind, ending in a nearly cir- 

 cular and deeply concave posterior face. On the posterior half 

 there is a thin but prominent hypapophysial keel, terminating in 

 a small tubercle. The articular surfaces for the atlas are usually 

 high and narrow, rising far up on the sides of the neural canal, 

 but their shape varies much in different individuals ; ordinarily 

 they are nearly flat, or very slightly concave from side to side, 

 the medial margin standing somewhat in advance of the external, 

 and slightly convex from above downward ; but sometimes the 

 medial margins are far in advance of the external, giving the sur- 

 faces a curiously warped appearance. Sometimes the surfaces 

 project down below the general level of the centrum, producing 

 a deep notch between the two ; in other cases they do not so 

 project, and there is no notch. The odontoid process varies 

 much in shape, being in some specimens short, broad, and 

 straight, with truncated free margin ; in others longer, nar- 

 rower, curved upward, and pointed ; but in all cases it is peg- 

 like, and either flat or convex on the dorsal side, very different 

 from the broad spout-like odontoid of the horse. The facet 

 upon its inferior surface is usually separated from the articular 

 faces of the centrum, but is sometimes continuous with them, 

 as is the case in the corresponding surfaces of the atlas. 



