90 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



spine has but a slight forward inclination. Of the zygapophyses, the 

 anterior ones are large, the posterior much smaller. 



As in Proterothermm, the number of sacral vertebrae probably exceeded 

 six, but only two are preserved, belonging to a very young animal with 

 milk teeth. The first sacral has a broad, extremely depressed centrum, 

 which narrows much posteriorly ; the neural canal is very much smaller 

 than that of the last lumbar, especially in the dorso-ventral diameter, and 

 the arch is separated by a considerable interspace from that of the second 

 vertebra, though this may be due to the immaturity of the animal, while 

 the spine is merely a vestigial crest. The anterior zygapophyses are 

 much reduced in size, though functional, and the posterior ones are 

 minute. The pleurapophyses are large and heavy, supporting the ilia 

 almost entirely, as the contact of these bones with the second vertebra is 

 very limited. On the anterior face of each pleurapophysis is a large, 

 prominent, convex and transversely oval facet for the concavity on the 

 transverse process of the last lumbar. 



The second sacral is much smaller than the first and it is evident that, 

 as in Proterothermm, the sacrum becomes exceedingly slender posteriorly, 

 with neural canal reduced to a minimum. 



No caudals are known, but the conformation of the sacrum renders it 

 almost certain that the tail was very short, as appears to have been the 

 case in all the known Santa Cruz representatives of this family. 



A few fragments of ribs indicate that the anterior ribs were compressed 

 and plate-like, though quite slender, and the posterior ones more rounded 

 and rod-like. 



Appendicular Skeleton. — Of the fore limb-bones, only a few fragments 

 have been preserved, the distal half of the humerus, the greater part of 

 the radius, the proximal and distal ends of the ulna and most of the manus. 

 Ameghino has, in addition, figured and partially described an almost com- 

 plete manus (94<5, p. 271, fig. 12), and thus a fairly adequate conception 

 of the structure of the fore limb may be reached. On the whole, the limb- 

 and foot-bones are much the same as in Proterotherium, but they are 

 relatively more elongate and much more slender and graceful, and there 

 are also certain other significant differences. 



The humerus (PI. XII, figs. 10, \oa) has a more slender shaft than in 

 Pyoterothermm ; the distal trochlea has the same simple, pulley-like char- 

 acter as in the latter, but the supratrochlear fossa is shallower and the 



