Mylodon harlani, from Rock Creek, Texas. 355 



Pelvis. 



At least four individuals are represented by their pelvic ele- 

 ments. Of these one, Cat. No. 10264, is essentially complete 

 except that it lacks the pubes and ischia. Two other sacra, 

 Nos. 10265 and 10266, are represented by three and five coos- 

 sified centra respectively, the latter being much the more com- 

 plete. There is also preserved a large portion of a left ilium 

 with its acetabulum, a right acetabulum, and a detached pelvic 

 symphysis, all arbitrarily referred to No. 10265. Two more 

 acetabula have been referred to 10266 and another left ilium 

 fragment with its acetabulum has been referred to No. 10267. 



The description will be based upon the complete pelvis No. 

 10264 and sacrum No. 10266 which gives additional data due 

 to its fractured condition. 



Sacrum. — This element in the Texas Mylodon consists of 

 ten coossified vertebrae of which the centrum of the most ante- 

 rior one is lost though the neural arch is still present. Of 

 these, according to Owen, the seven posterior vertebra? are true 

 sacrals, the anterior three being lumbars, the whole complex 

 forming a synsacrum. The coossification of the sacrum em- 

 braces not only the centra themselves but the neural arches, 

 the spinous processes of which are blended into a continuous 

 crest. In the fractured sacrum, No. 10266, the anterior first 

 lumbar centrum exhibits an almost circular articular face, the 

 convex outline being broken along the dorsal side where it is 

 depressed to form the floor of the neural canal. The sides of 

 the centrum, however, are compressed, the two faces meeting 

 on the mid-ventral line in a rather acute angle. In the second 

 lumbar vertebra the ventral aspect begins to broaden and 

 flatten in its posterior half, this flattening becoming a pro- 

 nounced concavity downward, bounded by slightly diverging 

 ridges, when the first sacral is reached. The second sacral 

 presents the rather curious anomaly of the almost total reduc- 

 tion in depth of the centrum, w T hich is merely a widely ex- 

 panded plate of bone, owing in part to the progressive increase 

 in size of the neural canal and the gradual rise of the ventral 

 limitation of the centra. A little material may have been lost 

 in the sacrum No. 10266 by abrasion, but at its anterior end 

 the second sacral centrum has a depth of 31*5 m,n , while at the 

 point of fracture, a little beyond mid-length, its thickness is 

 but 10 mm . 



Nutrient foramina pierce each centrum vertically and, as 

 Owen says,* "assist in the completely confluent state of the 

 bodies of the vertebrae in determining their true number.'' 



* Owen, R., Description of the skeleton of an extinct gigantic sloth, p. 04, 

 1842. 



