PROMERYCOCHCERUS 109 



Peterson (1914) described the skeleton of this species in considerable detail. In the following 

 description I have condensed his work or quoted directly therefrom. 



The vertebral formula is C 7, D 14, L 6, S 7-8, Ca 4 +. Merycoidodon has thirteen dorsals 

 and seven lumbars. In the atlas, 



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 more deeply separated below, and the posterior superior exit of the arterial canal has dis- 

 appeared. . . . On the internal posterior part of the arch, above the articulation for the axis is a large, round open- 

 ing, 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 is much shorter anteroposteriorly than that of Merycoidodon. 



The neural spine also overhangs more in front, while the articulation for the atlas extends further below the 

 odontoid process, and is divided from the pedicle by a deep round notch, which is sometimes bridged over. . . . 

 The posterior border of the neural spine is slightly excavated, and displays a broad transverse rugose surface, 

 terminating in the overhanging process. The transverse process is directed more outwardly and the entire bone is 

 more robust than in Merycoidodon. . . . The foramen for the vertebral artery is large and pierces the base of the 

 transverse process parallel with the long axis of the centrum. 



The third and fourth cervicals are very nearly alike. The centra are short, depressed, and 

 somewhat opisthoccelous, with a strong median keel. "The pedicle is low, the neural canal rather 

 small, and the superior surface of the arch rugose with heavy anterior and posterior zygapophyses." 

 The fifth cervical differs only in its longer spine, more delicate transverse processes, and larger 

 vertebrarterial foramen. The inferior median keel of the sixth cervical is less strongly developed 

 than on the preceding, and the centrum is more depressed, while the arterial canal is larger than that 

 of any of the other cervicals. The seventh cervical is characterized by its high, robust spine, its 

 strong depressed pedicles, and the presence of a small arterial canal, piercing the transverse process 

 at the base near the inferior border. The centrum is much depressed. 



The very large neural spine of the first dorsal is the heaviest in the vertebral column; other- 

 wise it is very similar to the seventh cervical. There are large capitular and tubercular facets, 

 separated only by a sharp bony ridge on the inferior surface near the base of the heavy transverse 

 process. The second dorsal has the highest neural spine of any vertebra, but its anteroposterior 

 diameter and width are less than those of the preceding vertebra. The capitular facets are larger 

 and deeper, but the transverse process is not so large as in the first dorsal. The third dorsal has a 

 smaller and more backwardly inclined spine and the transverse process is also smaller than in the 

 second dorsal. The fourth, fifth, and sixth dorsals are much alike. 



The centra gradually increase in length and weight, while they decrease in width. The ventral and lateral 

 surfaces increase in convexity fore-and-aft, and the transverse processes and neural spines decrease in size. The 

 summits of the spines are enlarged into rounded rugose knobs, in which the antero-posterior exceeds the transverse 

 diameter. 



In the seventh dorsal there is a tendency for the posterior intervertebral notch to become a 

 foramen. On the right side only the foramen is complete. The eighth, ninth, and tenth dorsals 

 are much alike. These vertebra; have distinct intervertebral foramina, the centra are long, narrow, 

 and high, with a sharp keel, and the transverse processes are shorter and the capitular and tubercular 

 facets are closer together. The eleventh dorsal 



is characterized by the rounded and interlocking postzygapophysis and the usually vertical or nearly anticlinal 

 neural spine. The prezygapophysial facet of this vertebra in the present species is quite small. The transverse 

 process is short and the ascending accessory process above the tubercular facet is more robust and roughened than 

 in the preceding vertebras. The intervertebral foramen is present. 



