No. i.] OSTEOLOGY OF PCEBROTHERIUM. 2J 



Professor Cope I find a minute canal through the base of the 

 transverse process, but this is probably an individual peculiarity. 



It will be plain from the foregoing description that the cervi- 

 cal vertebrae of PcebrotJierium have already assumed the features 

 characteristic of the Tylopoda, but in a somewhat less marked 

 degree than in the recent forms of that group. Indications of 

 a more primitive condition are found in the structure of the 

 transverse processes, the retention of the vertebrarterial canal 

 in the sixth vertebra, the shape of the atlas, the character of the 

 odontoid process, and the less elongated and less massive devel- 

 opment of the neck as a whole. If we take the length of the 

 humerus as a standard, and call that ioo, the length of the neck 

 in PcebrotJierium would be 197 ; in AucJienia, 280; and in Came- 

 lus, 286. 



The Dorsal Vertebrce. — The number of dorsals characteristic 

 of Poebrotherium cannot yet be definitely determined, as no 

 specimen has as yet been found with the series complete. In 

 the individual upon which this description is for the most part 

 founded, there are preserved twelve vertebrae of this region 

 and, in addition, a fragment of a centrum which probably repre- 

 sents a thirteenth, though it may perhaps belong to a lumbar. 

 The first dorsal has a centrum which is nearly as long as the 

 last cervical, and is broad and flattened from above downward ; 

 the anterior rib-facets are nearly flat, but are on projecting 

 processes, very much as in the llama. The inferior face of the 

 centrum has on its anterior part a broad ridge with raised and 

 rounded margins, and on each side of this a small pit, all of 

 which occur also in Auchenia, the first dorsal of which this ver- 

 tebra closely resembles, though its centrum is decidedly longer 

 in proportion. The prezygapophyses are larger, higher, and 

 project more in advance of the centrum than in the recent 

 forms ; the transverse processes, on the other hand, are rather 

 shorter, and end in a large, deeply concave facet for the tubercle 

 of the first rib. 



The second, third, and fourth dorsals have centra which are 

 shaped very much like that of the first, but they gradually be- 

 come less distinctly opisthocoelous, shorter, narrower, and less 

 depressed. In the middle region of the thorax the vertebra' 

 are relatively large and heavy, with large spines which slope 

 strongly backward ; but these spines are very much more slen- 



