No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 3 1 5 



centra, which are strongly keeled inferiorly, and have faces set 

 obliquely to the long axis ; the anterior face is hemispherical, 

 and the posterior deeply concave. In these three vertebrae the 

 centra are of nearly equal length. The neural arches are low 

 and very broad on top ; the zygapophyses are large, strongly 

 projecting, and present nearly vertically. The neural spine is 

 represented by a faint ridge on the third, which becomes a small 

 tubercle on the fourth, and a distinct spine on the fifth. The 

 transverse processes have considerable antero-posterior exten- 

 sion and become successively shorter and stouter as we pass 

 backward ; at the same time the inferior lamella, which is rudi- 

 mentary on the third, steadily enlarges on the succeeding ver- 

 tebrae. 



The sixth cervical (PI. XXII., Figs. 11 and 12) is shorter than 

 the preceding ones, and has no hypapophysial keel or tubercle ; 

 instead, the lower edge of the posterior face is emarginated in 

 the median line. The neural spine is quite high and inclined 

 strongly forward. The transverse process is a short, stout hook, 

 curved upward at the extremity, and the inferior lamella has 

 greatly increased in size, attached to the centrum throughout its 

 length, and projecting considerably both in front and behind it. 



The seventh, cervical is still shorter than the sixth; it has a 

 broad, flat, and depressed centrum, which, in advance of the 

 transverse processes, is somewhat suddenly constricted to form 

 the small, nearly hemispherical, and obliquely placed head ; the 

 posterior face is quite broad, and displays on each side of the 

 rounded pit for the head of the first dorsal a facet for the head of 

 the first rib. The transverse processes are quite stout and simple, 

 and are not perforated by the vertebrarterial canal. The prezyga- 

 pophyses are nearly flat, very prominent, and obliquely set, fac- 

 ing more inward than upward ; the postzygapophyses are less 

 conspicuous, as they do not project behind the neural spine. 

 The latter is stout, strongly inclined backward, and occupies 

 the whole antero-posterior extent of the neural arch, but unfor- 

 tunately none of the specimens is sufficiently complete to enable 

 us to estimate its height. 



Comparing the posterior five cervical vertebrae of the horse 

 with those of Mesohippus, we find many important differences. 

 The centra have been much elongated, so that the neck as a 

 whole is much longer than in the Miocene genus ; all of the 



