124 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



and less vertical, having a more outward inclination, so that, when seen 

 from below, the lamellae of the two sides are more divergent. 



The seventh cervical of Theosodon is much shorter than the sixth and 

 has a broader and more depressed centrum, especially behind, where the 

 width is increased by the large rib-facets ; the ventral keel is quite promi- 

 nent and much thicker than on the sixth vertebra, with a rough tubercle 

 at the anterior end. On the dorsal side of the centrum, forming the floor 

 of the neural canal, is a wedge-shaped prominence from each end, with a 

 depression on each side of it ; the two wedges do not meet in the middle 

 of the centrum and the concavities are connected between the apices of 

 the wedges. The neural canal is large, but the neural arch is quite narrow 

 and slender, being greatly reduced by the deep notches between each pair 

 of zygapophyses, which are more widely separated and the posterior pair 

 more strongly divergent than in any of the other cervicals. The trans- 

 verse processes are quite broad and depressed ; their full length is not 

 shown in any of the specimens, but was probably not great. A minute 

 canal pierces the base of the transverse process and may have transmitted 

 a branch of the vertebral artery, but is far too small to have contained the 

 main trunk of that artery. 



The number of trunk- vertebrae is not as yet determined beyond all 

 question, but, as we have already seen, was very probably 19, and their 

 division into regions varies slightly, as it does in existing mammals ; the 

 thoracic vertebras may be either 13 or 14, and the lumbars 6 or 5. 



Curiously enough, the first thoracic vertebra (Pis. XVII, fig. 5 ; XVIIl, 

 6) is remarkably like a cervical in appearance, and especially the seventh 

 of that series, from which it differs principally in the presence of an addi- 

 tional pair of facets for the heads of the second ribs and of facets for the 

 tubercles of the first pair on the transverse processes. So, there can be 

 no question as to the place of this exceptional vertebra in the series. The 

 centrum is shorter than that of the last cervical, with narrow anterior and 

 broader posterior face, and much less distinct ventral keel. On the dorsal 

 side of the centrum are wedge-shaped prominences which are like those 

 of the seventh vertebra, but are more nearly in contact at their apices. 

 The anterior rib-facets are very large, the posterior smaller and more con- 

 cave. The neural canal is large and the arch narrow, though broader 

 than that of the last cervical. Although the neural spine is broken in all 

 of the specimens, it obviously was short and slender and had a decided 



