106 



viz. the scapho-trapezial confluence, the Megatherium, like the Mylodon, re- 

 sembles the Sloth, not the Ant-eater. In the modification also by which the 

 hand of the Megathere differs from that of the Mylodon, viz. the rudimental 

 poUex, it is to the Sloths that it the more nearly approximates by that mutilation. 

 In the Orycterope, on the contrary, the thumb is represented by two very small 

 ossicles ; in the Myrmecophaga jubata, though it be the least of the five digits, it 

 is complete with all its phalanges. Only in the Sloths do we find the thumb 

 represented by a broad based metacarpal articulated by two points, one to the 

 trapezial process of the scaphoid, the other to the trapezoides and second 

 metacarpal. 



In the Mylodon the thumb is succeeded in the digital series by two large un- 

 guiculate digits, as in the Bradypus didactylus ; in the Megatherium it would 

 appear that it was followed by three large unguiculate digits, as in the Bradypus 

 tridactylus. The huge terrestrial predecessors of these small scansorial leaf- 

 devouring quadrupeds retained the two outer toes, minus their terminal phalanges, 

 it is true, yet of great size and strength, and modified expressly for the purpose 

 of supporting the ponderous body in terrestrial progression. The fore-foot of 

 the Mylodon thus exhibits the type of that of the Unau, with the superaddition 

 of the phalanges of the thumb, and of the two mutilated outer digits probably 

 imbedded in a modification of the integument analogous to a hoof : the fore- 

 foot of the Megatherium manifests the type of that of the Ai, with only this 

 essential difference, that the fifth digit, instead of existing as a rudiment, was 

 developed to those proportions which progression on the ground required. If, 

 however, the fourth digit terminated in the Megatherium, as in the Mylodon, 

 by two stunted phalanges, and was encased together with the fifth digit in a 

 hoof, the external resemblance of its fore-foot to that of the Unau, would, on 

 account of the atrophied poUex, be closer than in the Mylodon. 



With respect to the other existing Edentata, the Myrmecophaga jubata ought, 

 by reason of the clawless condition of its fifth digit, to immediately succeed the 

 Sloths as next of kin to the Megatherioid quadrupeds. 



A few remarks now remain to be offered in regard to the differences which 

 the bones of the fore-foot of the Mylodon present when compared with their 

 known analogues in the Megalonyx, Scelidotherium, &c. With regard to the 

 Megalonyx, which, from its correspondence in size with the Mylodon, and its 

 imperfectly-known dentition, might well be supposed to be generically identical. 



