No. I.] OSTEOLOGY OF PCEBROTHERIUM. 23 



arch shows on each side of the median line an articular surface 

 for the accessory facets on the basi-occipital : Pcebrotherium 

 agrees with Auchenia in showing no trace of this. The poste- 

 rior facets for the axis are high and narrow, and are reflected 

 upon the inner surface of the arch ; the facets of the two sides 

 have a somewhat more oblique position than in the recent 

 genera, where they lie in nearly the same transverse plane. 

 The transverse processes differ from those of the recent genera 

 in their much greater antero-posterior extension, as they are 

 produced considerably behind the condyles for the axis, and 

 terminate posteriorly in blunt points, as in the ordinary rumi- 

 nants ; from these points the thickened margins of the trans- 

 verse processes sweep forward in gentle curves, dying away 

 upon the anterior cotyli. Seen from the side, the edge of the 

 process is not sinuous, but nearly straight. In Auchenia the 

 transverse processes project hardly at all behind the facets for 

 the axis, and in the camel they terminate in advance of these 

 surfaces. In both of the living forms the edge of the trans- 

 verse process is bent in a sigmoid curve, running anteriorly and 

 dorsally. The foramina of the atlas are much as in the recent 

 genera. The neural arch is perforated by a foramen for the 

 superior branch of the first spinal nerve, and the forward exten- 

 sion of the transverse process converts the anterior notch at its 

 root into a foramen. The posterior opening of the vertebrarte- 

 rial canal enters the base of the transverse process from behind, 

 while in the camel and llama it is placed upon the dorsal side. 



The axis has already assumed a strikingly tylopodan charac- 

 ter. The centrum is very long, narrow, and depressed, and is 

 faintly keeled throughout its length, the keel terminating behind 

 in a high and quite rugose hypapophysial tubercle. The ante- 

 rior end of the centrum is quite suddenly expanded to form the 

 articular surfaces for the atlas, which rise high upon the sides 

 of the neural canal, and inferiorly are continuous with each 

 other and with the facet upon the ventral face of the odontoid 

 process. The lower edge of this compound facet is, however, 

 emarginated in the middle line, indicating that formerly the 

 surfaces of the two sides were separated, while in the existing 

 forms of both Pecora and Tylopoda this margin is uninter- 

 ruptedly curved from side to side. The odontoid process is pe- 

 culiar in being exceedingly short and semi-cylindrical in shape, 



