Mylodon harlani, from Rock Creek, Texas. 359 



anterior superior spinous process the outline is deflected 

 toward the acetabulum at an angle of about 80°, but the margin 

 is nearly straight and not so sinuous as in Owen's figure and 

 description. Medially the anterior margin is continued into a 

 sharp crest, deflected outward and slightly downward to form 

 the superior limitation of the sacro-ischiadic foramen. Irregu- 

 lar radiating ridges on the surface of the ilium mark the ori- 

 gin of the huge gluteal muscles which must have been of 

 great importance when the animal reared into the semi-erect 

 posture. The ilium is very thin in the center of its expanse, 

 so that the bone has been broken away here for a distance of 

 several inches in the center of each ilium. 



The acetabula are large, well buttressed, and capable of 

 resisting the thrust of the thigh in both the quadrupedal and 

 semi-erect postures. The Harderian depression for the lodg- 

 ment of the round ligament lies, as Owen has described it, on 

 the side of the acetabulum which is next to the obturator 

 foramen and increases in depth as it decreases in width. Its 

 linear extent, however, is variable from less than one-third the 

 width of the socket measured upon the curve in the complete 

 pelvis to nearly half, as in Owen's specimen, in the pair of 

 acetabula No. 10266. The termination is very deep and irregu- 

 lar in the latter pair and relatively shallow in the pelvis No. 

 10264. 



Of the pubes only the basal portions remain, forming, 

 though the sutural limitation is no longer in evidence, the 

 lower portion of the acetabulum and extending downward in 

 a relatively slender flattened process of bone which terminates 

 on either side about 75 mm from the margin of the articular sur- 

 face of the acetabulum. It does not differ in any marked 

 detail from Owen's figures of M. robustits. 



The ischia are even less evident in all of the material, as 

 practically only that part which enters into the acetabulum is 

 present. This bears, however, a thin ridge of bone inclining 

 somewhat obliquely inward to form the ischiadic tuberosity 

 which is confluent with the sacrum and bounds posteriorly the 

 sacro-ischiadic foramen. No trace of the ventral limb of the 

 ischium is present. There is a well-preserved pelvic symphy- 

 sis, to which the number 10265 has been assigned. This is of 

 much less antero-posterior extent than in Owen's figure and 

 also differs from it by showing a prominent reinforcing ridge 

 running transversely on the inner aspect. Yentrally the two 

 sides meet in an angle of about 90°. Anteriorly this angle is 

 prolonged into a rugose process, the inner surface of which is 

 smooth and concave, the sharp limiting margin being continu- 

 ous with the anterior edges of the pubes. Posteriorly the 

 ischial margins are sinuous in that they approach, swing paral- 



4m. Jour. Sct.— Fourth Series, Vol. XXXIX, No. 232.— April, 1915. 

 24 



