24 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 



than the fourth and has a much depressed and transversely arched cen- 

 trum, with concave posterior face which is extended out upon the trans- 

 verse processes ; the pedicles of the neural arch are low and narrow, 

 bringing the zygapophyses of each side very close together ; the neural 

 arch is extremely slender, leaving a wide space between itself and the 

 arch of the succeeding vertebra ; the transverse process is quite distinctly 

 divided into dorsal and ventral portions and each of these is again divided 

 into anterior and posterior processes, both of which function as additional 

 zygapophyses, articulating with the corresponding processes of the fourth 

 and sixth vertebrae. 



In neither specimen is the sixth or seventh cervical preserved. 



Of the thoracic vertebrae the first and second and the last five are pre- 

 served in connection with one specimen, and the third and fourth with the 

 other, but the nine vertebrae do not form a continuous series and the region 

 must have had at least eleven. In general the thoracic vertebrae resemble 

 those of Tatti but they are relatively heavier and there are some differences 

 of detail. 



The first thoracic has a broad and depressed centrum, like that of the 

 cervicals in shape, but much more elongate and narrower posteriorly ; the 

 neural canal is quite large and the spine very long, broad, laterally com- 

 pressed and much heavier than that of Tatu. At the upper end this spine 

 is coossified with that of the second vertebra, but this is doubtless merely 

 an individual peculiarity. The spine is strongly inclined backward with 

 reference to the long axis of the centrum, but owing to the curvature of 

 the vertebral column in this region it has a nearly erect position. The 

 prezygapophyses, which are of the cervical type, are large, widely separated 

 and placed on the pedicles of the neural arch but little above the level of the 

 centrum, while the postzygapophyses are of the thoracic type and situated 

 on the ventral side of the neural arch. Although they are both broken away, 

 it is evident that the transverse processes were very large and, as in Tatu, 

 extended forward, enclosing and almost concealing the last cervical. 



The second thoracic is similar to the first, but has a somewhat thicker 

 centrum of more uniform breadth, a much smaller, transversely oval neural 

 canal, prezygapophyses placed on the neural arch, and a higher, somewhat 

 more slender and less steeply inclined spine ; the transverse processes are 

 broken on this vertebra also, but enough remains to show that they were 

 less massive than on the first. 



