102 



497- The left humerus. This bone presents a large convex oval head, on each 

 side of which is a tuberosity for the implantation of the supra- and sub- 

 scapular muscles : these tuberosities do not rise above the articular con- 

 vexity so as to restrict the movements of the shoulder-joint, as in the 

 Solipedia and Ruminantia, but exhibit a structure and disposition con- 

 formable to those which characterize the proximal extremity of the 

 humerus in the Mylodon, and those Mammalia which enjoy rotatory and 

 lateral movements of the fore-limb. The tuberosities are, however, re- 

 latively more developed and give greater breadth to the proximal end of 

 the humerus in the Scelidothere than in the Megathere. 



The distal end of the humerus although mutilated clearly indicates that 

 it had the same characteristic breadth of the external and internal 

 condyles as in the Megatherium. The left condyle was perforated for the 

 direct passage of the artery or median nerve, or of both, to the fore-arm. 

 The groove for the musculo-spiral nerve on the outer side of the 

 humerus is overarched at its upper part by a strong obtuse process ; 

 which is comparatively less developed in the Megatherium. The trochlea 

 or inferior articular surface of the humerus supports, as in the Megathe- 

 rium, two well-marked convexities with an intervening concavity ; this 

 indication of the rotatory power of the fore-leg is confirmed by the form 

 of the head of the radius. 



498. The left ulna. 



499. The left radius. These bones of the Scelidotherium leptocephalum are 



cemented together by the calcareous matrix of the stratum in which 

 almost the entire skeleton of the same individual was found. 



The relative length of the fore and hind extremities cannot be precisely 

 determined from the present imperfect skeleton of the Scelidothere ; but 

 there is good evidence for believing that the fore extremity was the 

 shortest. 



The humerus is shorter than the femur by one-ninth part of the latter 

 bone ; and the radius, which wants only the distal epiphysis, must have 

 been shorter than the humerus. In the Scelidotherium the general pro- 

 portions of the radius much more nearly approach those of the Mylodon, 



