CHARTER XItl 
RODENTIA 
W8HILE all of small size, numerically the rodents make 
about a third of our collection, the number of genera 
and species being, however, relatively small. All are 
hystricomorphs with the pattern on the crowns of the 
teeth relatively simple. While the incisors are typically 
rodent-like, permanently growing teeth, the molars are 
all rooted, some being entirely brachydont, others begin- 
ning to show hypsodont features. 
So far as yet known, the rodents make their first appear- 
ance in South America, in this Deseado formation. Were 
they, as Ameghino thought, developed there from such a 
form as Propolymastodon or Promysops of the Casamayor 
formation? Or did they migrate into Patagonia from 
some other section? For the former proposition to be 
convincing to me, it would require more complete material 
of the forms suggested than now exists.* Other groups 
of hystricomorphs occur in the Theridomyidae of the 
European Oligocene, and from the Oligocene of the Fayum. tf 
Either the old world forms are descended from the South 
American forms, or vice-versa. The two African lower 
jaws are very much like those of Cephalomys, and my 
feeling is that the Patagonian forms are derived from some 
immigrant reaching that section before Deseado times. 
The Deseado genera are not widely different from each 
-other, but it is evident that they are the representatives 
of at least two families, and my expectation is that other 
families will be found eventually to be already represented. 
* 1 have a lower jaw of Propolymastodon which, though not complete in 
front, gives me no suggestion that the incisor was rodent-like, and I am 
inclined to think that the incisor associated with the type of P. carlo-zitelli 
is a mistake. 
T Osborn, Bul. Amer. Mus. Nat. Hist., Vol. 24, p. 265. 
