Aprii, 2, 1894.] 



KNOWLEDGE 



75 



of the anterior teeth. Indeed, it would require only a 

 still greater elongation and narrowing of the skull of a 

 scelidothere, coupled with the total loss of the teeth, to 

 produce one very similar to that of an anteater. 



So far as we are aware, palreontologists have not yet 

 been able to trace a complete transition from the gigantic 

 ground-sloths of the Pleistocene deposits of Buenos Aj-res 

 to their diminutive representatives from the older Tertiary 

 deposits of Patagonia, although it is known that some of 

 the species from the intermediate forms were inferior in 

 point of size to their more recent allies. It is, however, 

 very interesting to find that the pigmy ground- sloths of 

 these Patagonian deposits had transversely-ridged pris- 

 matic teeth like those of the megathere, and not the 

 cylindrical or ti-iangular ones of the mylodonts, scelido- 

 tiieres, and megathere ; thus apparently indicating that 

 the former type of tooth is the oldest. The contrast 

 between the pigmy ground-sloth and the giant ground- 

 sloth (Megatherium) is. however, most remarkable. The 

 total length of the skeleton of the former was only about 

 three feet ; while its skull was less than six inches, whereas 

 the skull of the megathere was over a couple of feet in 

 length. Then, again, the whole series of live upper teeth 

 occupy in the pigmy ground-sloth a space of less than an 

 inch and a half, or less than the diameter of a single tooth 

 of its gigantic relative. That such a diminutive creature, 

 if as naked and undefended as its huge cousin appears 

 to have been, needed some special protection, is pretty 

 evident ; and it is the need of such defence from attack 

 that has led us to suggest that the creature may have 

 lived in holes excavated by its powerful claws. 



Leaving for a moment the mutual relationships and 

 affinities of all these dift'erent animals, we have to direct a 

 short glance at the skeleton of the body and limbs of the 

 ground-sloths. In the first place, this difi'ers from that of 

 the sloths in the shortness and extreme massiveness of the 

 limbs, and especially in the extraordinary stoutness and 

 width of the bones of the hind leg and pelvis. In the 

 general form of the scapula or blade-bone, and more 

 especially in the presence of a complete pair of clavicles or 

 collar-bones, the ground-sloths resemble the sloths and 

 differ from the anteaters ; the clavicles of the latter being 

 rudimentary. The skeleton of the fore foot is, however, 

 essentially that of an anteater, the inner toe being 

 rudimental. the next three, and more especially the middle 

 one, being enormously enlarged and furnished during life 

 with huge claws, while the outermost was small and 

 clawless. That during life the creature rested on the 

 outer side of this fifth claw and the backs of the three 

 large ones, in anteater fashion, may, fi'om the structure and 

 arrangement of their bones, be considered certain. UnUke 

 the anteater, in which, as we have seen, it rests upon the 

 sole, the hind foot of the Pleistocene groimd-sloths is 

 even more strangely modified than the front one, these 

 creatures walking only on its outer edge, while the 

 enormous middle toe, with its gigantic claw, does not 

 appear to have touched the ground in walking and was 

 thus always kept sharp. The first toe is wanting, and the 

 second rudimentary, while the two outer ones were rela- 

 tively small and unpro\'ided with claws. Some idea of the 

 gigantic proportions of the megathere may be gathered 

 from the circumstances that its hind foot measures nearly 

 a yard in length. Of the pigmy ground-sloths of Patagonia 

 the complete skeleton has not yet been described ; but so 

 far as our recollection of a specimen in the La Plata 

 Museum carries us, we believe that it was not of the 

 extremely specialized type characterizmg the gigantic 

 forms. Moreover, while in the larger forms the terminal 

 joints of the feet were neither grooved nor split at the 



extremities, in the small Patagonian species these were 

 deeply cleft at the end, after the fashion obtaining in the 

 scaly anteaters or pangolins of India and Africa. As 

 regards the structure of the vertebral column, the ground- 

 sloths exhibit certain peculiarities distinctive of the ant- 

 eaters, and which are only rudimentary in the sloths. 



When we add to the foregoing brief survey of the chief 

 structural peculiarities of the skeleton of the extinct 

 edentates forming the subject of the present article the 

 circumstance that, from their enormous size, they must 

 necessarily have been terrestrial in their habits, we are in 

 a position to realize the appropriate nature of the term 

 ground-sloths by which they are designated. These 

 creatures may, in fact, be briefly described as edentates 

 having a skull, teeth, and shoulder-girdle very similar to 

 those of the sloths ; while as regards their back-bone and 

 feet they come very close to the anteaters, although in 

 the later and more gigantic forms the specialization 

 characterizing the fore feet of the latter has been extended 

 to the hinder pair. 



We now come to the interesting question of the mutual 

 relationships and phylogeny of the three groups of eden- 

 tates discussed in the course of the foregoing paragraphs. 

 Now, in the first place, we shall have little hesitation in 

 regarding the pigmy ground-sloths, which are the earliest 

 known representatives of the group, as the direct ancestors 

 of the gigantic megathere. A modification in the struc- 

 ture of the teeth would equally well permit of their having 

 likewise been the ancestors of the mylodonts, which, as 

 we have seen, possess sloth-like teeth. This, however, 

 will not permit us to regard the mylodonts as having been 

 the forerunners of the sloths, seeing that the latter have 

 a less specialized type of hind foot ; and we must 

 accordingly regard the sloths as being a side-branch 

 derived from the pigmy ground-sloths or some nearly 

 allied form after the acquisition of cylindrical teeth, but 

 before the hind foot had acquired the specialization 

 characterizing the mylodonts and megatheres. Hence the 

 curious structural similarity between the front teeth of 

 some of the mylodonts and the two-toed sloth must be 

 another instance of that parallelism in development to 

 which a special article has already been devoted. 



With regard to the anteaters, we have already seen 

 that the fore foot of these animals resembles that of the 

 pigmy ground-sloths in that the terminal joints of the 

 larger toes are marked by a longitudinal groove repre- 

 senting the cleft of those of the latter ; and as in both 

 groups the middle toe is the largest, there is no reason 

 why the anteaters should not trace their origin to these 

 same pigmy ground-sloths or a closely allied type. In 

 this case the specialization has resulted in a lengthening 

 of the skull and the loss of the teeth, the hind foot having 

 retained more or less of the primitive type. Here, like- 

 wise, we must notice that the resemblance presented by 

 the skull of the scelidotheres to that of the anteaters 

 must be regarded as an instance of parallel development. 



From the structure of their teeth, the ground-sloths 

 were evidently vegetarians ; and the same may be said 

 of the sloths, which are animals specially modified for the 

 exigencies of a purely arboreal existence. On the other 

 hand, the anteaters, as their name implies, have given up a 

 vegetable diet and taken to living on ants, and to this may 

 be attributed their total loss of teeth. Should germs of 

 teeth ever be found in their jaws during an early stage of 

 existence, we venture to predicate they will approximate 

 in structure to the teeth of the ground- sloths. 



Thus ends our survey of the structure and relationships 

 of the extraordinary monsters forming the subject of this 

 article ; but we cannot conclude without saying a few worda 



