186 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 Pert- 
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 
