CHAPTER XIII 



RODENTIA 



While 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.f 

 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. 



* I 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. 



fOsborn, Bui. Amer. Mu*. Nat. Hist., Vol. 24, p. 265. 



