III.] MARSUPIALS. 107 



Brazilian caverns ; while Nothrotherium of the same deposits 

 seems to have been another nearly related form with teeth of the 

 Megalotherium type. The imperfectly known Nothropus of the 

 Pampean and Ortotherium of the Parana beds seem, on the other 

 hand, to be late survivals of another group typically represented 

 by the genera Eucholaops and Pseudhapalops of the Santa Crucian 

 epoch of Patagonia. These forms differ from all those noticed 

 above in that the terminal joints of some of the toes have a 

 median cleft as in the great anteater, and likewise in the 

 elongation of the metatarsal bones ; and it seems probable that the 

 hind foot was not so much everted as in the later representatives 

 of the family. The skull is of the general type of that of Mylodon ; 

 most of the molars being prismatic in form, and surmounted by a 

 pair of transverse ridges, more or less closely connected at their 

 extremities so as to produce an oval cavity on the grinding surface. 

 The first tooth is, however, tusk-like, and separated by a gap from 

 the others. In some of these early ground-sloths the skull did 

 not exceed three inches in length ; but other species were con- 

 siderably larger. They are evidently generalised types, and were 

 probably nearly allied to the ancestral stock which gave rise to 

 Mylodon and Megalonyx, if indeed they be not the actual pro- 

 genitors of both. 



The last group for consideration is that of the marsupials, or 

 pouched mammals, among which the family of the 

 opossums (Didelphyidce), with the three genera 

 Didelphys 1 , Dromiciops, and Chironectes, is now confined to the 

 New World, the great majority of the numerous species being 

 Neogaeic, although the common opossum (Didclphys marsupialis} 

 ranges from Chili and Brazil to the United States. Although 

 certain forms from the Santa Cruz beds described under the 

 names of Eodidelphys and Prodidclphys were originally assigned to 

 the present family, these have been subsequently identified t^y 

 Dr Ameghino 2 with the under-mentioned family of the Microbio- 

 theriidce. True opossums occur, however, in the Monte Hermoso 



1 This genus is divided into several sub-genera, regarded by some writers as 

 entitled to generic rank. 



2 Bol. Ac. Cordoba, Vol. xm. p. 363 (1894). 



