No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 3 1 7 



but little exceeds the vertical ; posteriorly the centra become 

 much broader and more depressed. The spines appear to be 

 thin and compressed, the transverse processes quite stout and 

 with considerable antero-posterior extent ; on the last (PI. XXII., 

 Fig. 16) and penultimate vertebrae the transverse processes have 

 articular surfaces for contact with those of the succeeding verte- 

 brae. The zygapophyses are interlocking in a moderate degree. 

 The obliquity of the faces of the dorsal and lumbar vertebrae 

 show that the back was decidedly arched. 



In the horse the back is nearly straight and the dorso-lumbar 

 vertebrae vary less in length in the different regions ; the lum- 

 bar vertebrae in particular are shorter in proportion than those 

 of Mesohippus. The spines are much more massive and more 

 nearly straight, and the zygapophyses of the posterior dorsal 

 and lumbar regions are much more decidedly of the interlocking 

 type. 



The first sacral vertebrae of Mesohippus has a very much 

 depressed centrum ; the pleurapophyses are greatly expanded 

 and form the large iliac surfaces, the second vertebrae not contrib- 

 uting to their formation. On the anterior edge of the pleura- 

 pophysis is a very large convex facet for the transverse process 

 of the last lumbar. 



No caudal vertebrae are preserved in the specimens. 



Nothing is known of the sternum, and the few fragments 

 preserved indicate that the ribs were more slender in proportion 

 than are those of the recent horses. 



V. The Fore Limb. 



The scapula is not well preserved in any of the specimens, 

 but enough remains to indicate it's chief characters. The glenoid 

 cavity is nearly circular and quite shallow, and is slightly 

 notched at its antero-internal border by the synovial incision. 

 The coracoid process is very large, both in the vertical and 

 antero-posterior directions, but is thin and plate-like, and is not 

 recurved at the free end. The neck is very narrow and long ; 

 the spine rises somewhat nearer to the glenoid cavity than in 

 the horse, and is not median in position, but nearer to the 

 coracoid border, thus making the postscapular fossa larger than 

 the prescapular. None of the specimens has a sufficiently well- 



