598 LAND MAMMALS IN THE WESTERN HEMISPHERE 



ending circle. The animal species which has once been put 

 into a train oiF motion is still probably moving in that train. 

 For, if one link in nature's chain might be lost, another and 

 another might be lost, till this whole system of things should 

 evanish by piece-meal." 



The tglyptodonts were also southern in distribution, and only 

 very imperfect remains of them have yet been recovered from 

 the North American Pleistocene, quite sufficient, however, to 

 make the identification certain. 



There were several genera of rather small fground-sloths in 

 the Pleistocene of Cuba. The best known of these, ^Megalocnus, 

 had several peculiarities of structure, but was plainly a member 

 of the fMegalonychidse. The ancestors of this genus probably 

 invaded Cuba in the Pliocene, when the island was joined to 

 Central America. 



Suborder fGRAViGRADA. fGrRouND-SLOTHS 



As the tground-sloths would appear to have had a more 

 central position within the order than any of the other groups, 

 our study of development may well begin with them. In the 

 Pleistocene there were three families of these gigantic brutes, 

 which ranged through the western hemisphere from Pennsyl- 

 vania and California to Patagonia. Unfortunately our know- 

 ledge of the developmental stages within the different families 

 is very unequal, and it is therefore impracticable to do more 

 than sketch the changes of the suborder as a whole and in a 

 general way. In the successive geological stages the pro- 

 portionate representation of the different phyla varied greatly ; 

 in the South American Pliocene and Pleistocene the fMylo- 

 dontidae and fMegatheriidse were the abundant forms, while 

 the fMegalonychidse were but scantily represented. In the 

 Santa Cruz Miocene, on the other hand, the overwhelming 

 preponderance was with the fMegalonychidse, the other two 

 families being comparatively rare and incompletely known. 

 From the still more ancient formations, the material so far 



