EDENTATA OF THE SANTA CRUZ BEDS. 205 



Ungual, digit I, length 026 Ungual, digit III, length 060 



" " " proximal width on " " " proximal width 020 



" " thickness 015 " " " " thickness... .028 



" " II, length 049 Digit I, length 057 



" " " proximal width 013 " II, " 097 



" " " thickness . . . .018 " III, " 102 



Restoration (Plate XXX). As there is no available material for a resto- 

 ration of Megalonyx or Notkrotherium, Owen's well-known figure of My- 

 lodon ('42, PI. I) has been selected as a standard of comparison and the 

 skeleton of H. longiceps has been drawn in a similar attitude. Between 

 the Santa Cruz and the Pampean genus the contrasts are much more 

 numerous and striking than the resemblances. In the former the head is 

 relatively small, with elongate rostrum, slender, rod-like premaxillaries 

 and long, pointed mandibular beak; the neck is long and light, and the 

 trunk, especially the thoracic region, is extraordinarily elongate, a very 

 marked difference from the rather short and extremely bulky trunk of 

 Mylodon; the neural spines of all the trunk-vertebrae are far shorter than 

 in the latter ; the tail, though heavy, is not nearly so massive. The pelvis 

 has a very different appearance, owing to the much less extreme ex- 

 pansion, eversion and rotation of the ilia and to the shorter pubes and 

 descending processes of the ischia. The limbs are far more slender than 

 in Mylodon, the feet pentadactyl and very little modified, all the digits 

 being functional and provided with claws ; although the manus may have 

 already acquired, in an incipient degree, the rotation which brought the 

 ulnar border to rest upon the ground in walking, it is probable that the 

 pes was simply plantigrade. 



On the other hand, this skeleton has a number of suggestive resem- 

 blances to that of the recent Tardigrada. In making such a comparison, 

 it should be remembered that the latter are extremely specialized and 

 adapted to a very unusual mode of life ; the whole skeleton is modified in 

 accordance with the habit of suspending the weight of the body and the feet 

 are transformed into mere hooks. In spite of such mechanical differences 

 between the strictly arboreal Tardigrada and the doubtless as strictly 

 terrestrial Gravigrada, there is much to suggest a common origin of the 

 two groups, and these correspondences are particularly clear in the skel- 

 eton before us. The type of skull and dentition is essentially the 

 same, and the extremely elongated trunk is very like that of Chofapus; 

 the tail is, of course, very different, but this is sufficiently accounted for 



