near Tinogasta, Catamarca. 117 



and Otro Cerro "^ we have here only a single form o£ Graomys, 

 all the specimens of this fine series being practically identical 

 in size. But a rather considerable number of them are 

 immature. 



4. Phyllotissp.i?). 



? . 834 (young). 

 Too young for determination. 



5. Akodon alterus^ Thos. 



c?. 841, 859, 878, 890; ? . 839, 843, 8G7, 887, 897. 

 Very similar to our series from Otro Cerro. 

 No. 878, quite young, shows that ?n' has a well-defined 

 anterior median groove at its tip. 



6. Octomys mimax, gen. et sp. n. 

 S . 850, 848 (young) ; ? . 838, 845, 880. 



Octomys t, gen. nov. (Octodontidse). 



General external cliaracters and shape of skull about as in 

 Octodontomys] molar structure approximately as in Aconcemys, 

 of which it is probably a non-fossorial rock-inhabiting 

 representative. 



External characters very much as in Octodontomys, and, 

 indeed, as in many other rock-inhabiting dry-conntry Muridae, 

 such as the larger gerbils — the light colour, whitish under 

 surface, large ears, and long tufted tail all being characters 

 of such animals. Indeed, the resemblance to Octodontomys 

 gliroldes is almost complete, so that even as species the two 

 niight readily be mistaken for each other. Structure of ears 

 and feet very much as in the older known animal, though tho 

 granulation of the soles is a little less coarse. 



All these characters present a striking contrast to those of 

 the one Octodont genus with more or less similar teeth — 

 liamely, Aconfemys (^Schizodon of Waterhouse), — which is a 

 fi'ssorial short-eared, long-clawed, dark-bellied, and short- 

 tailed form, as difi'erent as possible from the present animal. 



Skull also with the general shape of that of Octodontomys^ 

 less like that of Octodon, wholly unlike tiiat of the fossorially 



• Cf. Ann. & Majr. Nat. Hist. (9) iii. p. 494 (1919). 



t Not only does this animal belong to the group whose dental formula 

 gave rise to the name Ocfodrm, but its molars themseh es have a patteru 

 nearly re«embliug the figure 8. 



