ALLEN: mammalia; OCTODONTID.'E. 33 



Desmarest, both names having practically the same basis, namely, the 

 Vizcacha of Azara. As I have elsewhere shown,' the Argentine Viscacha 

 must apparently be called Viscaccia chilensis Oken. 



This species is unrepresented in the present collection, its range not 

 extending below the Rio Negro (latitude 41° south), and it thus barely 

 reaches the northern border of the region here considered. 



It has been so fully described and figured by Brookes, Waterhouse and 

 others, that a detailed account of it seems uncalled for in the present con- 

 nection. An extended notice of its habits has been given by Darwin, and 

 later by Hudson, as observed by them on the pampas of Buenos Aires. 



Family OCTODONTIDyE. 



Of the Octodonts only the Octodontinas reach Patagonia, and of the 

 five commonly recognized existing genera of this subfamily only one, 

 Cfeiiomys, appears to have been found east of the southern Andes. The 

 other four — Aconcemys, Spalacopus, Abrocoma and Octodon — occur 

 within or on the western slope of the Andes, and are not, as now known, 

 numerously represented in species. Ctenomys, on the other hand, is 

 characteristic of the plains and pampas, ranging from Tierra del Fuego 

 northward to southern Brazil and Bolivia, and westward into the base of 

 the Andes. It is a plastic and prolific group, swarming in favorable locali- 

 ties, and readily susceptible to changes of environment. At least some 

 twenty-five named forms are at present tentatively recognized, mostly on 

 rather slight differences of size or color. Although the extreme phases 

 are widely separated, it is probable that so many links still remain in the 

 chain of intergradation that when the group comes to be more effectually 

 known many of the forms now treated as species will be found to merge, 

 and that some of the names stand for very little that is tangible. Sev- 

 eral extinct forms have also been distinguished, the group extending back 

 into the Pleistocene, and according to some authorities to the Pliocene. 



The family Octodontidas, as at present constituted, forms a very hetero- 

 geneous assemblage, comprising groups that might well be assigned to 

 three distinct families. There is, for example, no close relationship be- 

 tween the Octodon-Ctenomys series and the Spiny Rats [Loncheres-Echimys 

 series), or of either of these with Ctenodactylits. At different times each 



' Proc. Biol. Soc. Wash., XV., 196, Oct. 10, 1902. 



