Peterson: The Osteology of Promerycochcerus. 



167 



The vertebral column of the type is complete to the caudals and 

 lies undisturbed upon the matrix in which it was found. The caudal 

 vertebrae are poorly preserved in all the material, only the proximal 

 region and other scattered bones being available. Inasmuch as the 

 type is partially imbedded in its original matrix, No. 1047 will be used in 

 preparing a general description of the parts not accessible in the type. 



The Atlas (Fig. 4). — The atlas of P. carnkeri presents some well- 

 marked characters, which differ from those of the same bone in Mery- 

 coidodon cidbertsoni. The transverse process is proportionally heavier 

 than in Merycoidodon, the anterior part of the ventral surface of the 

 process is more deeply excavated for the obliquus capitis muscle, the 

 cotyles for the occipital condyles are more concave, and niorc deeply 



4 5 



Fig. 4. Lateral view of atlas of Promerycochaenis carrikeri No. 1047; -3- nat. size. 

 Fig. 5. Lateral view of axis of Promerycochcerus carrikeri No. 1047; -3 nat. size. 



separated below, and the posterior superior exit of the arterial canal 

 has disappeared, 8 while further forward at the base and on the superior 

 face of the transverse process is usually found a small venal foramen 

 (See Fig. 4). On the internal posterior part of the arch, above the 

 articulation for the axis is a large, round opening, which undoubtedly 

 functions as the arterial canal, and is apparently characteristic of 

 this species. The rugose neural spine occupies a considerable area on 

 the top of the arch. 



The Axis (Fig. 5). — The axis is unusually shortened antero-pos- 

 teriorly when compared with that of Merycoidodon. The neural spine 

 also overhangs more in front, while the articulation for the atlas 



8 The presence or absence of this canal is a matter of individual variation in the 

 Oligocene genus. In the mounted skeleton of Merycoidodon cidbertsoni (No. 1391) 

 in the Carnegie Museum this canal is present, as is also true of some specimens 

 which Professor Scott studied (Morphologisches Jahrbuch, Vol. XVL 1890, p. 322), 

 while Dr. Wortman {Bull. Amer. Mus. Nat. Hist., Vol. VH. 1895, p. 149). did not 

 find a true canal in the material at hand, when he wrote his description. 



