198 PATAGONIAN EXPEDITIONS: PAL/EONTOLOGY. 



outward ; the gluteal surface is moderately concave and the iliac crest 

 somewhat thickened and rugose. In the huge Gravigrada of the Pleisto- 

 cene the ilia are immensely expanded and everted, making the gluteal 

 surface posterior rather than dorsal and the iliac surface anterior rather than 

 ventral, while the crista is enormously thick. However, the difference from 

 the Santa Cruz type of pelvis, obvious as it is, is not at all fundamental, 

 for the great expansion and eversion of the ilia occur in many unrelated 

 groups of very large terrestrial mammals, such as the elephants, the titano- 

 theres, etc., and the change. in the form of the pelvis from the Santa Cruz 

 to the Pleistocene ground-sloths is strictly analogous to that between 

 Palceosyops and Titanotherium. 



The ischium is much shorter than the ilium, but the disproportion is not 

 so great as in Bradypus ; it is depressed and broad, especially behind, 

 where it is ankylosed with the transverse processes of the last two sacral 

 vertebrae ; the sacro-sciatic foramen is quite small, but relatively larger 

 than in Mylodon, and the tuberosity is larger and more prominent than in 

 the latter. The pubis is slender and laterally compressed, expanding 

 somewhat at the distal end, where it forms a short symphysis ; it is rela- 

 tively much shorter than in Mylodon and the pelvic opening is conse- 

 quently smaller, especially in the dorso-ventral diameter; the obturator 

 foramen has approximately the shape of an equilateral triangle. The 

 acetabulum presents directly outward and is of elongate oval shape ; the 

 articular surface is quite narrow, because of the very large sulcus for the 



round ligament. 



MEASUREMENTS. 



Pelvis, length 262 Ilium, greatest width 108 



" greatest width 340 Ischium, length 101 



Ilium, length 161 Acetabulum, antero-posterior diameter. .050 



The femur (Plate XXXII, fig. 5) resembles that of Nothrotherium in 

 its proportions. The head is small and set upon a distinct neck and it 

 presents more directly inward, less upward than in Megalonyx, and rises 

 very little above the great trochanter ; the pit for the round ligament is a 

 shallow, pyriform depression and is entirely enclosed in the articular sur- 

 face. The great trochanter is a heavy, rugged mass, which rises higher 

 proximally than in the Pleistocene genera ; the digital fossa is very small, 

 but not so much reduced as in Megalonyx ; the second trochanter is more 

 prominent than in the latter, but less rugose and less extended distally, 



