262 ANNALS NEW YORK ACADEMY OF SCIENCES 



vide supra), but only the armadillos have maintained any foothold in 

 the northern world until modern times and these only in the southwest 

 corner of the Sonoran region, 'i'he anteaters and tree-sloths might be 

 expected to have originated in Patagonia and to have been driven north- 

 ward to tropical South America in accord with the theory of climate and 

 evolution here advocated. The geologiqal record, however, has failed to 

 show any certain evidence of this, and, as the Patagonian record is a com- 

 paratively full one, this fact should be counted as evidence that climatic 

 change is not the only causal factor of evolution. We must suppose, if 

 the record be adequate, that these groups originated and evolved in tropi- 

 cal South America. The annadillos are an extremely persistent group, 

 and the record gives no really convincing evidence of a Patagonian dis- 

 persal center, although it might be so interpreted. 



Glyptodonts and ground-sloths appear in the Pliocene and Pleistocene 

 of North America. The Pleistocene genera except Megalonyx are closely 

 allied to the genera of the Pampean formation, in part identical there- 

 with {Bracliyostracon, ? Glyptodon, Clilamydotheriuiii, Megatherium, 

 Megalonyx, Nothrolherium, Mylodon). These, or allied genera equiva- 

 lent in specialization, inhabited South America from Ecuador to Pata- 

 gonia in the late Pliocene and Pleistocene. The only genera found in the 

 Pliocene of North America are Megalonyx and Glyplotherium, decidedly 

 more primitive and are best interpreted as earlier forerunners of the main 

 invasion which appeared at the beginning of the Pleistocene. Mylodon 

 has been recorded from the Blanco beds of Texas, but this is an error. 



MAESUPIALIA 



Marsupials are at present almost limited to the Australian and Austro- 

 malayan region, where, in the absence of placental mammals, they have 

 diversified into a wide variety of size, habits and adaptation, paralleling 

 the adaptive radiation of the higher mammals in the northern continents. 

 A single unspecialized group, the opossums, representing quite nearly the 

 primitive type from which all marsupials are derivable, survives in the 

 Neotropical region, one or two of its species ranging northward into the 

 Sonoran subregion of Holarctica. Another primitive survivor in the Neo- 

 tropical region is the rare little Ccenolestes, formerly regarded as a primi- 

 tive member of the diprotodont marsupials, but now considered to be of 

 polyprotodont affinities, its diprotodont resemblances being due to paral- 

 lelism. 



What we know of the paleontology of the order is in complete accord 

 with the theory of their being primarily of northern origin, their dispersal 

 preceding that of the early placentals. 



