1 86 THE DESEADO FORMATION OF PATAGONIA 



Our material does not permit the discussion of the 

 skeleton or even of the skull as a whole, for the specimens 

 occur only as isolated jaws, palates, or even as isolated 

 teeth. In a few cases, the upper and lower dentitions 

 are associated, but in no case was skeleton material clearly 

 associated with the teeth. The remains look very much 

 like such as are often found today in the western United 

 States under a hawk's nest or below the roosting place of 

 owls. I think most of our specimens passed, before burial, 

 through the stomach of birds or carnivors. 



Ameghino puts most of the forms in the family Cepha- 

 lomidae, which he considers ancestral to Hystricomorpha 

 in general. I feel, however, that it is better to assign 

 the Deseado genera to the families which have persisted 

 until recent times, as Scott and Ameghino, in another place, 

 have done. There are six living families, four of which 

 Scott found already represented in the Santa Cruz. Two 

 of these clearly may be continued back into the Deseado, 

 the Erethizontidae, and the Chinchillidae, nothing as yet 

 having been found to represent the Santa Cruz families 

 Cavidae and Octodontidae. 



Chinchillidae 



In the Deseado, this family is represented by the genera 

 Cephalomys, Scotamys, and possibly Litodontomys. Cepha- 

 lomys is very abundant and seems to be ancestral to Peri- 

 mys of the Santa Cruz; Scotamys is relatively rare but 

 seems to be ancestral to Scotaeumys; while Litodontomys 

 is also rare and as far as I can see without a successor. 



Cephalomys Ameghino 



Cephalomys Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 494. 



This is the common genus of the Deseado, over three- 

 fourths of the specimens of rodents found belonging to 

 one of its three species. Its dental characters mark it 



