136 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



has a very short peduncle, which is deep dorso-ventrally, but laterally 

 compressed and thin, and which expands rapidly into the very broad iliac 

 plate. This plate is less strongly everted than in the Pampean genus and 

 the division into dorsal and ventral processes is less distinctly marked. 

 Another difference from Macraitchenia is that the ventral process is wider 

 and has a much less thickened border. On the inner side of the dorsal 

 process is a large, rugose depression for the attachment of the sacrum, the 

 process rising well above the latter and concealing it in side-view. On 

 the outer side, the gluteal surface is smooth and but slightly concave. The 

 pubic border of the ilium is very indistinct and speedily dies away upon 

 the inner side. The acetabulum is large and has a very prominent 

 anterior border, and a deep, conspicuous sulcus for the round ligament. 



The ischium is quite short and stout, trihedral for most of its length, 

 but expanding posteriorly into a thin plate. The spine and sciatic notch 

 are feebly marked, more distinctly in some individuals than in others, 

 while the tuberosity is very strongly developed, being prominent and rugose, 

 though not nearly so long as in Macrauchenia. The pubis is short and 

 heavy, especially in the symphysial region, the anterior part of which is 

 very thick. As a whole, the symphysis is longer than in the last named 

 genus. On the anterior border of the pubis, near the acetabulum, is the 

 prominent and rugose spine. The obturator foramen is an elongate oval, 

 with its principal axis in the antero-posterior direction, while in Macrau- 

 chenia it appears to be transverse. 



The femur (PL XX, figs. 2, 3, 3a) is long and very much like that oi Mac- 

 rauchenia, but considerably lighter and more slender. The head is sessile 

 and rather flattened and does not rise to the level of the great trochanter, 

 while in the Pampean type it is large and hemispherical and projects 

 prominently above the trochanter. The sulcus for the round ligament is 

 a broad and deep emargination of the articular surface on the posterior 

 aspect of the head and not visible in front-view. The great trochanter is 

 very high, heavy and rugose, rising proximally into a blunt point, which 

 extends above the level of the head, and is recurved so as to enclose a large, 

 deep digital fossa. In Macratichenia the great trochanter is much reduced 

 in height, but is very massive and rugose. The second trochanter on the 

 femur of Theosodon is quite small, forming a low, thin ridge, and placed 

 quite far below the head. The third trochanter also has a low position 

 on the shaft, nearly midway between the two ends, and projecting pos- 



