GLIRES OF THE SANTA CRUZ BEDS. 387 



Loncherinae have no doubt descended from those of Santa Cruz times, 

 but the latter are not yet sufficiently well known for us to distinguish the 

 ancestral from the collateral forms. It is surprising that Ctenomys, now 

 one of the most abundant and characteristic of Patagonian rodents, should 

 have no Santa Cruz representative. 



Another group of Santa Cruz genera comprises certain specialized 

 types which did not persist to a later epoch. Prominent among these are 

 Acaremys and Sciamys, which Ameghino has very properly grouped to- 

 gether as a subfamily of the Eretkizontidce. The numerous species of 

 these two genera are all very small and some of them minute, superfi- 

 cially like the rats and mice whose place they filled in Santa Cruz 

 times, when South America possessed no representatives of the Myomor- 

 pha. In these animals the molar pattern is like that of the tree-porcu- 

 pines, but the skull, limbs and feet are quite different and indicate that 

 their habits were probably terrestrial. 



Still a third category of genera, already alluded to, includes those 

 rodents which, though not ancestral to any of the existing types, are yet 

 phylogenetically important, because they have preserved, with more or less 

 modification, the ancestral forms of. an earlier epoch. Neoreomys is an 

 example of this ; on the whole, it resembles Myocastor and Capromys 

 more than any other recent genera and is therefore included in the same 

 subfamily, but the likenesses to the Dasyproctidce and Dinomyidce are 

 also very suggestive and indicate that this genus is not far removed in 

 structure from some earlier and more generalized form, which was the 

 common ancestral stock of several distinct families. Another instance of 

 the same kind is afforded by Perimys, the Santa Cruz species of which are 

 not ancestral to any modern rodent, or to any yet known from intermediate 

 formations, but they probably differ little from some earlier genus that 

 formed the common stock whence all the Chinchillidce were derived. 



Although most and probably all of the existing South American families 

 of Hystricomorpha had already become distinctly separated as such, and 

 even the subfamilies are more or less clearly defined, yet the separation 

 is not so great as between the existing representatives of the same groups. 

 There is more uniformity in dental pattern and in the structure of the 

 skull and skeleton than we find at present, and the various families are 

 evidently converging backward in time to a common ancestry. This is 

 strong evidence for the unity of origin of all the Hystricomorpha, or, at 



