M. Lund expressly states that it is produced by a different structure and arrangement of the tarsal bones, from that which exists in the Sloth, but he does not specify the nature of this difference. If the astragalus, which I have referred with doubt to the _Megalonyx_, do not actually belong to that genus, it is evidently part of a very closely allied species. Now this astragalus, as we have seen, resembles most closely that of the Megatherium; and since we may infer that the calcaneum, scaphoides, and cuboides had a like correspondence, the inclination of the sole of the foot inwards must have been very slight, as I have determined from examination of the structure and co-adaptation of those bones in the incomplete skeleton of the Megatherium in the London College of Surgeons. Such an inclination of the foot may be conceived to have facilitated the bending of the long claws upon the sole, during the ordinary progressive movements of the animal, but it is quite insufficient to justify the conclusion, that it related to an application of the hind-feet for the purposes of climbing. It is not without interest again to call to mind the deviation of the structure of the astragalus of the Scelidothere from the Megatherioid to the Dasypodoid type of structure. For if the Megatherioid type of structure had really been one suitable to the exigencies of climbing quadrupeds, it might have been expected to have exhibited the scansorial modifications more decidedly, as the species diminished in stature; but as regards the instructive bone of the hind-foot, the modifications of which we have just been considering, this is by no means the case. DESCRIPTION OF A MUTILATED LOWER JAW OF THE MEGALONYX JEFFERSONII. In the preceding section an astralagus was described, which was regarded as belonging possibly to the same Edentate species as the jaw figured and described, p. 69, Pl. XVIII. and XIX., under the name of _Mylodon Darwinii_; but the same correspondence,—that of relative size,—renders it equally possible that this astragalus may belong to the species of _Megalonyx_ to which the lower jaw now under consideration appertains. There could be no doubt, from its structure, that it was the astragalus of a gigantic species of the order _Bruta_, and of the _Megatherioid_ family, and more nearly allied to the Megathere than is the Scelidothere, but sufficiently distinct from both.