Femur, length 630 mm. Femur, greatest proximal diameter 240 mm. Femur, distal diameter 170 mm. Tibia, length 370 mm. Tibia, greatest proximal diameter 164 mm. Tibia, greatest distal diameter 114 mm. Astragulus, antero-posterior diam. 123 mm. Astragulus, transverse diam. 114 mm. Astragulus, height 65 mm. 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 beginning to show hypsodont features. So far as yet known, the rodents make their first appearance 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.[20] Other groups of hystricomorphs occur in the _Theridomyidae_ of the European Oligocene, and from the Oligocene of the Fayum.[21] 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. [Footnote 20: 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.] [Footnote 21: Osborn, Bul. Amer. Mus. Nat. Hist., Vol. 24, p. 265.] 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. 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 carnivores.