292 THE OSTEOLOGY OF ELOTHEBITTM. 



by the immense weight and length of the head. Among recent artiodactyls Hippopotamus 

 has cervical vertebra? most like those of Elotherium, though there are many differences 

 in the details of construction. The most apparent of these differences lies in the greater 

 and more uniform height and thickness of the neural spines in the modern genus. 

 Doubtless the even more exaggerated massiveness of the skull in the latter is the occasion 

 of this increased development of the cervical spines. In Sus the perforation of the neural 

 arches for the passage of the spinal nerves constitutes an important difference from 

 Elotherium. 



The thoracic vertebrae would appear to have numbered thirteen, though this point 

 cannot, as yet, be determined with entire certainty, and while the thoracolumbar vertebrae 

 were, in all probability, nineteen in number, as is well-nigh universal among the artio- 

 dactyls, yet there were doubtless variations in the number of ribs, as is very frequently the 

 case among existing animals. 



The first thoracic lias a rather small centrum, with decidedly convex anterior and 

 nearly Hat posterior face : the facets for the rib-heads are very large and deeply 

 concave. The transverse process is rather short, but very large, heavy and rugose, and 

 bears an unusually large, concave tacit for the tubercle of the first rib. The prezyga- 

 pophyses are of the cervical type, but present more obliquely inward than in the vertebra? 

 of the neck, while the postzygapophyses are, as in the other thoracics, placed upon the 

 ventral side of the neural arch. The neural canal is high and narrow and its anterior 

 opening has assumed a cordate outline. The neural spine is inclined strongly backward, 

 much more so than that of the seventh cervical, and though laterally compressed it is 

 extremely high, broad and massive, greatly exceeding in all its dimensions that of the 

 last neck vertebra. 



The anterior six thoracic vertebra- (see PI. XVIII, Fig. 5) are very much alike in 

 appearance. The first three have broader and more depressed centra, which in the others 

 become deeper vertically and more trihedral in section. The transverse processes are 

 very large and prominent and carry large, deeply concave facets for the rib tubercles. 

 The neural spines are very high, thick and heavy, and are strongly inclined backward, 

 with club-shaped thickenings at the tips. At the seventh thoracic begins a rapid reduc- 

 tion in the length and weight of the -pines, a process which reaches its culmination on 

 the eleventh vertebra, which has a remarkably short, weak and slender spine. This 

 arrangement results in a great hump at the shoulders, somewhat as in Titanotherium, 

 though in a less exaggerated form. In both genera, the length of the anterior thoracic 

 spines should be correlated with the great elongation and weight of the skull which 

 requires immense muscular strength in the neck and shoulders. Hippopotamus has no 

 such hump, but this is probably explained by its largely aquatic habits. 



