24 SCOTT. [Vol. V. 



the inferior surface being convex and the superior flattened, and 

 the free anterior margin is gently rounded, not abruptly trun- 

 cated. In the existing genera of the Camelidce, as well as in the 

 true ruminants, the odontoid process is spout-like, with deeply 

 concave dorsal surface, while that of Pcebrothcriiim resembles 

 rather the odontoid of Anoplotherium, the Oreodontidcz, and other 

 primitive selenodonts, except that it is decidedly smaller. The 

 anterior end of the axis in PcebrotJierium differs from that in 

 Camelus in being much less oblique, and in the fact that the 

 odontoid is not directed dorsally as well as forwards : in these 

 respects it is more like the axis of the llama. The posterior 

 face of the centrum is slightly concave. The neural arch is high 

 and narrow, and the neural spine is expanded into a high thin 

 plate, which is very different in character from the spine of the 

 recent genera, in that it is both higher and longer (antero-poste- 

 riorly), projecting behind the postzygapophyses, while in the 

 llama, and especially in the camel, it terminates some distance 

 in front of the zygapophyses, ending in a bifid rugosity, which is 

 not indicated in Pcebrotlieriinn. The postzygapophyses are small 

 and slightly concave from side to side. The transverse processes 

 are broken away in all the specimens which I have had the oppor- 

 tunity of examining, but it is evident that they were short and 

 very thin, depressed plates. The anterior opening of the ver- 

 tebrarterial canal is placed laterally, as in the camel (but not so 

 far forward) ; in the llama the opening is inferior rather than 

 lateral. This canal appears to enter the neural canal a little 

 more than half-way toward the posterior end, though it is diffi- 

 cult to decide this question with accuracy from the available 

 specimens. The pedicels of the neural arch are perforated by 

 the second pair of spinal nerves. 



The third cervical is slightly longer than the axis, and has a 

 narrow, depressed centrum which broadens posteriorly, so that 

 the hinder face is much broader than the front : the latter is 

 slightly convex, the former concave. Throughout the entire 

 length of the centrum there is an inferior keel, which, for the 

 anterior two-thirds of its course, is a faintly marked ridge, but 

 then becomes much more prominent and ends behind in a 

 rugose tubercle. The transverse processes expand posteriorly 

 and terminate in flat, deposed wing-like projections. There is 

 already an indication of a division of the process into anterior 



