151 



The hind-foot of the Mylodon, partly by the position and shape of the upper 

 articular surface of the astragalus, partly by the articulations of the metatarsal 

 with the cuboid and cuneiform bones, is slightly inverted, so as to rest upon its 

 outer edge, and receive the superincumbent weight upon that edge, and chiefly 

 upon the two external toes : these are peculiarly adapted to that office by their 

 singularly massive proportions, their form, and their mode of articulation*. The 

 metatarsal of the fifth toe, by the rough, honey-combed surface of its broad 

 outer and lower facet, bespeaks the thick and callous integument with which it 

 was shod. 



The weight sustained by the astragalus and calcaneum is transmitted to the 

 fifth metatarsal in one direction immediately by the os cuboides, and in another 

 direction by the astragalus, through the medium of an arch formed by the 

 scaphoid, the external cuneiforme, the produced distal end of the middle meta- 

 tarsal, and that of the fourth metatarsal : this powerful bone, by that part of its 

 hoofed extremity which extends beyond the outer toe, likewise aids in sustaining 

 the weight transmitted to it by the foregoing arch, and also more directly by the 

 OS cuboides. By this admirable arrangement the two unguiculate toes were ab- 

 solved from all participation in the service of sustaining the animal while stand- 

 ing or walking upon the ground, and are left free for such other offices as it 

 might have had occasion to apply them to. In the Feline animals the claws are 

 kept sharp and serviceable by the mechanism for their retraction backwards 

 upon the padded proximal phalanges : in the Mylodon the same effect was gained 

 by their oblique inflection, and by the concentration of the superincumbent 

 weight on the two outer toes in the manner shown by the figure of the right 

 hind-foot in Plate I. 



Dr. Lundf derives his strongest argument that the claws of the hind-foot 

 were apphed exclusively for grasping, and that the Megatherioids were therefore 

 climbers, from this inversion of the sole ; but the advantages which thereby 

 accrued to the Mylodon in the preservation of the non-retractile claws from 

 the habitual wear and tear of ordinary locomotion, appears to be the legitimate 

 physiological explanation of that partial inversion, the true insight into the uses 

 to which the claws were applied being gained by a study of the other peculiari- 

 ties of the skeleton. 



* Plates XXI. and XXII. f Loc. cit. p. 20. 



