81 



Humerus*. — The humerus of the Mylodon surpasses every other bone of the 

 extremities in the strong development of all those inequalities which bespeak 

 the volume and force of the attached muscles. 



The tuberosities, though not elevated so as to interfere with the rotation of 

 the head of the bone, as in the large ungulate quadrupeds, are broad and well- 

 defined. The rough deltoidal tract covers a great proportion of the fore-part of 

 the shaft of the bone, from which it is separated by a more or less prominent 

 marginal ridge. The pectoral ridge is well-marked. The musculo-spiral impression 

 presents an unusual width and depth. The supra-condyloid plates are very 

 broad and prominent. The trochlear articulation for the bones of the fore-arm 

 indicates, by the regular convexity which it presents to the radius, the free rota- 

 tion of that bone. 



In these general characters the humerus of the Mylodon resembles that of the 

 Megalonyx and Megatherium ; but it differs in its greater strength, the shaft 

 being relatively much thicker, with a stronger deltoidal platform, and the proxi- 

 mal tuberosities are larger and higher, particularly the external one. The hume- 

 rus of the Mylodon differs in a more marked respect from that of the Megalonyx, 

 in the absence of the perforation of the internal condyle, which in the Mylodon is 

 simply notched at its upper part by the brachial nerve and vessels, as in the 

 Megatherium ; but the notch is less strongly marked in the more gigantic species. 

 The form of the articular surface for the ulna offers another well-marked distinc- 

 tion between the Mylodon and the Megalonyxf . In the latter species, and like- 

 wise, to judge from the figures of Bru and Pander, in the Megatherium, this part 

 of the distal articular surface of the humerus is convex in every direction ; in 

 the Mylodon it is only convex, and that in a very slight degree, from before 

 backwards, and is concave from side to side. 



The proximal articular surfaceof the humerus in the Mylodon is sessile, regularly 

 convex, of an elliptical form, with the long axis parallel with the antero-posterior 

 diameter of the bone. In the Megalonyx, and apparently also in the Mega- 



* Plates XI. XII. XIII. fig. 1. 



t The coBQparison here pursued is founded on the cast of the humerus of the Megalonyx, disco- 

 vered in the great depository of the Mastodon's remains called Big-bone-Iicli, Ohio, and described by 

 the accomplished anatomist and naturalist Dr. Harlan, in his instructive ' Medical and Physical 

 Researches,' p. 325, pi. 13. fig. 10. 



