EDENTATA OF THE SANTA CRUZ BEDS. 363 



family is concerned. For the Megalonychidce and Planopsidce, the number 

 of well preserved skeletons and parts of skeletons already collected is so 

 large as to make the negative evidence very strong. If the ancestors of 

 the Gravigrada had dermal ossifications, these ossifications had been lost 

 in all except one family before the Santa Cruz epoch. It is of course 

 possible that the dermal armor was independently acquired by the Mylo- 

 dontidce and at a comparatively late period. Materials are lacking for a 

 definite decision between these two alternatives, though perhaps the least 

 unlikely assumption is as follows : That the single ancestral group of 

 American edentates possessed dermal ossifications which were not exposed 

 on the surface or covered merely with epidermal scales, but concealed 

 within the skin, which was covered with hair; that in the Loricata they 

 gradually assumed the forms so familiar in the glyptodonts and armadillos, 

 while in the other orders they were lost, except in a single family of the 

 Gravigrada. 



In the absence of the ancestral forms of the true sloths and the ant- 

 eaters, it is difficult to reach any definite conclusions concerning the second 

 great branch of the American edentates, though it is possible to draw a 

 number of probable inferences from a study of the Santa Cruz Gravigrada. 

 As has been shown in detail in the preceding pages, the skeleton of these 

 animals is very much less specialized than it became in their Pleistocene 

 successors and, what is of particular significance, the Santa Cruz ground 

 sloths are very much closer to the ant-eaters in structure than are those 

 of the Pampean beds. Aside from the skull, all parts of the skeleton, 

 the vertebral column, the limb-girdles, limbs and feet display this resem- 

 blance in a very marked manner, and it seems hardly open to question that 

 the Vermilingua and the Gravigrada had a common ancestry, a conclusion 

 which was long ago reached by Flower ('76) though upon somewhat dif- 

 ferent grounds. The skull is, of course, very different in the two groups, 

 but the differences are not of such a kind as to preclude the common origin 

 of both. 



Even more closely connected are the Gravigrada and the Tardigrada ; 

 the resemblance is most clear in the dentition and skull, but there are also 

 many points of likeness throughout the skeleton. That the skeleton of 

 the Tardigrada should be extremely peculiar, is not surprising in view of 

 their very unusual habits ; the weight of the body is stispended from the 

 limbs, not carried upon them in the ordinary fashion, and thus the me- 



