Mylodon harlani, from Mock Creek, Texas. 367 



outer side. The longitudinal ridges are present on the rear 

 aspect of the radius in both instances, but they vary slightly in 

 their position. 



Two ulnse are present, a left and a right, but as they vary 

 in size they can not have pertained to the same animal. I 

 have therefore assigned to the right the catalogue number 

 10265, and to the left, No. 10266. Of these, unfortunately 

 the former is the least perfect, though neither is present in its 

 entirety. The ulna is a roughly triangular bone, widest at 

 the proximal end instead of at the distal as in the case of the 

 radius, trihedral in section, with an enormously developed, 

 inwardly bent olecranon. The proximal sigmoid facet is very 

 large and consists of two areas, a slightly concave one 

 articulating with the inner condyle of the humerus, and a 

 deeply concave portion which embraces the rear part of the 

 outer condyle. Below this latter area is a deep excavation which 

 receives the proximal end of the radius and within which it 

 rotates. There is a rounded notch confluent with this excava- 

 tion which extends up into the outer humeral facet and which 

 doubtless lodged a well developed ligament. This is present, 

 though in variable degree, in both ulnae from Texas and also 

 in one of the Perkins casts from Oregon, but is not visible in 

 Owen's figures of Mylodon robustus. Nor does Sir Richard 

 mention it. This notch would seem therefore to be distinctive 

 of the North American forms. On the inner aspect, the ulna 

 is smooth and concave, the concavity being bounded proximally 

 by the inwardly bent olecranon, posteriorly by the inwardly 

 deflected hinder margin, and proximo-anteriorly by the rim 

 of the humeral facet. The outer aspect of the bone bears a 

 prominent ridge, apparently, as Owen says, parallel with the 

 hinder margin. This ridge arises from the posterior rim of 

 the humeral articulation and passes toward the front margin of 

 the bone as it moves distalward. Posterior to the ridge the bone 

 is deeply concave, especially behind the humeral articulation. 

 Distally, however, this area becomes slightly convex as in 

 M. robustus. None of the distal extremity is preserved in 

 either ulna, but the Oregon cast seems to show the bone in its 

 entire length, although even here the distal end was broken. 



I can see no marked distinction between the Texas and 

 Oregon ulnse other than superior size and a greater rugosity in 

 the northern specimen, which may well have been the result of 

 greater individual age, as in the case of the humeri. There is 

 quite as much difference in the character of the humeral facet 

 of the two Texan specimens Nos. 10266 and 10265 as between 

 10265 and the Oregon cast, though the variable extent of 

 abrasion and other mutilation to which the Texas material has 

 been subjected renders a precise proportional study difficult. 



