51 



rises from the body, and incloses a spinal canal almost as large as the body 

 itself: its superior anterior margin overhangs the body*, and supports the 

 posterior expansion of the arch of the preceding vertebra, to which it is articu- 

 lated by two large elliptical and nearly horizontal articular surfaces f. This 

 imbricated overlapping of the arches of the dorsal vertebrae prevails thi-ough- 

 out the greater part of that region. The anterior part of the base of the spinous 

 process commences between the anterior articular surfaces and extends to the 

 posterior part of the arch, increasing in thickness. The spines are slightly in- 

 clined backwards throughout the dorsal region, and maintain the same antero- 

 posterior extent to their thick truncated and rugged summits. They present 

 very little difference in length, but gradually gain in antero-posterior diameter 

 and thickness from the first to the eighth vertebrse. A somewhat short but very 

 thick and strong transverse process extends outwards and a little upwards from 

 the upper and posterior part of the neural arch|, and terminates in a nearly 

 circular and flat surface §, looking obliquely downwards and outwards, for the 

 tubercle of the rib. On the upper part of the transverse process there is a 

 stout tubercle, and between this and the base of the spine, a thick longitudinal 

 ridge, both of which are shown in fig. 3. These eminences are most developed 

 on the middle and posterior dorsal vertebrae, and indicate the great power of the 

 spinal muscles. In the last dorsal vertebra the small rib was anchylosed by both 

 its head and tubercle, resembling a long transverse process, perforated at its 

 base I . The neural arch of this vertebra was anchylosed to that of the first lum- 

 bar vertebra ; but the centrum remained free, and was characterized by a very 

 flat posterior surface. 



The three vertebrae in the lumbar region being confluent with each other and 

 the sacrum, of which they thus form part, as in the class of Birds, will be de- 

 scribed with the rest of the anchylosed region of the spine. 



Ribs. — ^There are sixteen pairs of ribs, of which nine were articulated with the 

 sternum, by means of completely ossified cartilages. The ribs increase in length 

 to the seventh, maintain the same length to the thirteenth, and then rapidly di- 

 minish, the last being the shortest and straightest. They are relatively broader 

 than in the ungulate quadrupeds of the same size as the Mylodon, equalling, in 



* PI. VIII. fig. 4. t Figs. 2 and 3. d. J Fig. I. § Fig. 2. c. || PI. X. fig. 1. a. 



g2 



