364 PATAGONIAN EXPEDITIONS I PALEONTOLOGY. 



chanical character of the stresses is altered and what is normally compres- 

 sion becomes tension. The entire bony structure is modified in accord- 

 ance with this most exceptional arrangement and yet, in spite of this 

 great modification, there are many notable resemblances between the 

 modern Tardigrada and the Santa Cruz Gravigrada, so many as to render 

 it altogether probable that both orders were derived from a single 

 ancestral type. 



The backward convergence in time of the edentate orders is even more 

 conspicuously shown by the families within those orders. All the Santa 

 Cruz glyptodonts are very closely similar and the only reason for group- 

 ing them into distinct families is because they seem to show, in an incip- 

 ient and inconspicuous way, the first stages of those differences which 

 became so very much more distinct in the subsequent epochs. Similarly, 

 the three families of the Gravigrada were, in Santa Cruz times, very closely 

 alike to one another, differing only in details of a very minor kind. 

 Had the ground-sloths become extinct at the end of the epoch, instead of 

 persisting till a much later time, no division into families would seem to 

 be necessary, for the differences are significant only as foreshadowing the 

 far more important differences of subsequent epochs. 



In the case of the armadillos the evidence is less satisfactory, for the 

 Santa Cruz beds have as yet yielded the probable ancestor of only a single 

 modern genus, while those of other recent families and genera must, it 

 seems likely, be sought in a different region. Taken as a whole, how- 

 ever, the testimony of the Santa Cruz fauna to the unity and close rela- 

 tionship of all the American edentates, is very cogent. 



