224 PATAGONIAN EXPEDITIONS.' PALAEONTOLOGY. 



the ribs, but on the last thoracic that for the head of the last rib is like 

 a step cut in the base of the transverse process ; short anapophyses are 

 given off from the hinder borders of the transverse processes. The postzy- 

 gapophyses of the last three thoracics and the prezygapophyses of the 

 last two have the semicylindrical form characteristic of the lumbar region ; 

 the accessory postzygapophyses first appear on the nineteenth thoracic, as 

 small, ventrally facing facets, overlapping the accessory prezygapophyses 

 of the twentieth vertebra, which are carried upon small, shelf-like pro- 

 jections from the base of the principal pair of prezygapophyses. 



The lumbar vertebrae have broad and depressed centra, with two large 

 foramina opening upon the ventral side of each ; the neural spines are some- 

 what longer than those of the posterior thoracic region, but rather narrower 

 and more erect ; the transverse processes are broad, depressed and thin, 

 but of no great length ; the metapophyses diminish in size posteriorly and 

 on the second lumbar are smaller than those of the last three thoracics ; 

 the zygapophyses and accessory articular processes are like those, already 

 described, of the posterior thoracic region, and no additional ones appear 

 upon the lumbars. 



The sacrum consists of five vertebrae, all of which are large, though 

 decreasing in size posteriorly ; the centra are broad and much depressed 

 and are slightly concave on the ventral side, with ventral foramina on the 

 first, fourth and fifth ; the neural canal is wide and low ; the foramina for 

 the passage of the sacral nerves are large, especially those between the 

 second and third vertebrae. The first sacral has well developed prezyga- 

 pophyses and the last bears much smaller postzygapophyses ; those of the 

 other vertebrae are indicated by roughened tubercles, and the neural spines 

 are fused into a continuous ridge. Even in quite young individuals no 

 division of the transverse processes into dia- and pleurapophysial ele- 

 ments is apparent ; the first and second sacrals have large processes for 

 articulation with the ilia, and the third has smaller processes, which touch 

 the ilia only by the anterior border, while those of the fourth do not reach 

 the pelvis and those of the fifth are connected with the ischia. 



As no specimen with a complete tail has yet been found, the number 

 of caudal vertebrae is uncertain, but as in the other species of the genus, 

 was probably about twenty, and they are relatively less heavy than in the 

 large species. The first caudal has a short, broad and thick centrum and 

 very elongate transverse processes, which are narrow and of only moder- 



