118 Mr. 0. Thomas o?? Mammals from 



specialized Aaoncemys, Muzzle slender. Supraorbital edges 

 sharp, not beaded, without postorbital processes. Inter- 

 parietal region without ridges, even in old specimens. A large 

 supra-meatal island visible on the top of the skull. Palatal 

 foramina short and broad, with a broad median septum. 

 Mesopterygoid fossse narrow, pointed anteriorly, reaching 

 forward to the level of the middle of m^. Bullae very large, 

 larger than in any other member of the group. 



Teeth. — Incisors of medium strength, more opistliodont 

 than in any of the allied genera (again making a parallel to 

 certain Geibillines), the incisive angle, as measured from the 

 surface of the tooth-row, about 76°, but, owing to the hyper- 

 trophy of the bullse, they appear still more turned in backwards 

 in relation to the general lines of the skull. 



Molars rootless, their pattern strikingly like that in 

 Aconcemys (figured by Waterhouse *), the three anterior teeth 

 above and below 8-shaped, with two subequal lobes. M^ and 

 Wg with the posterior lobe reduced. A capsule appearing on 

 the outer side of the jaw at the root of m^. 



Genotype. Octomys mimax, sp. n. 



The Octodontinse — if we accept the obviously correct 

 exclusion of Ahrocoma from them advocated by Miller and 

 Gidley t — fall into two groups, one with crescentic and tiie 

 other with S-shaped teeth. Each of these groups has, again, 

 fossorial and non-fossorial members, Octodon and Octo- 

 dontomys being the non-fossorial members of the crescent- 

 toothed group and Ctenomys the fossorial, while in the other 

 group Spalacopus and Aconcemys are both fossorial, and the 

 present new genus now supplies to it a non-fossorial repre- 

 sentative. 



This handsome and remarkable animal, so strikingly like 

 Octodontomys externally but so different from it essentially, is 

 one of the most striking novelties that I have had the oppor- 

 tunity of describing from Argentina, and Sr. !^idin is to be 

 congratulated on so tine a discovery. 



Octomys mimax, sp. n. 



Size and general external appearance about as in Octo- 

 dontomys gliroides. Colour of body above pale drabby buff. 

 Under surface white, the hairs along the median area white 



* Nat. Hist. Mamm. ii. pi. viii. fig. 4 (1848). 

 t Joui-n. Wash. Acad. Sci; viii. p. 445 (1918). 



