146 Mr. M. A. C. Hinton on 



the primitive features found in Eothenomys with peculiarities 

 of its own ; and in certain respects its skull differs from 

 these of both Eothenomijs and Carijomys. In my opinion 

 all three should be regarded as distinct genera, of which 

 the leading characters are shown in the following "key'' : — 



MammaB 0—2=4 ; sole-pads 6. 



Cheek-teeth rootless. 



Eony palate simple, essentially as in Evotoim/s. 



Auditory bullae without internal spongy tissue. 



A. Interorbilal region of skull wide, usually con- 



cave transversely between the temporal 

 ridges ; the latter marginal, not approaching 

 each other or the middle line in old age. 



a. Ml without closed triangles, its inner and 



outer dentinal spaces forming more or less 



confluent transverse pairs Eothenomys, Mill. 



b. Ml as in true Microtus, with five substan- 



tially closed and alternating triangles .... CaryomySf Thos. 



B. Interorbital region of skull narrower ; the 



temporal ridges approximating to form an 

 incipient interorbital sagittal crest in old 

 age. Ml substantially as in Eothenomys .... Anteliomys, Mill. 



These three genera are of considerable interest^ since they 

 appear to be quite closely related to Evotomys, and they 

 have, in my opinion, descended from, a stock which may well 

 have been directly ancestral to Evotomys also. The close 

 affinity is betrayed by the form and structure of the skull, 

 which in Eothenomys and Caryomys is almost exactly like 

 the skull of Evotomys ; in one small matter only does the 

 skull of Caryomys go a little further— the edges of the post- 

 palatal pterygoid shelves bend down to join the margin 

 of the palatal shelf upon its dorsal surface"^. The skull of 

 Anteliomys is a little more highly specialized ; the anterior 

 portion of the temporal muscle appears to have become 

 slightly stronger, so that the temporal ridges tend to approxi- 

 mate and form a median crest by fusion in the interorbital 

 region, thus foreshadowing the modifications met with in 

 that region in the higher voles ; and in adults a median 

 spine, which lies, however, in the same horizontal plane as 

 the rest of the palate, is developed upon the hinder edge of 

 the palatal shelf. 



In one respect the present group of genera shows itself 

 to be a little more primitive than Evotomys. In the least 

 specialized species of Eothenomys^ and to a lesser extent in 

 Caryomys, tlie cheek-teeth, particularly m^, m^, and m^, are 

 more complex, retaining in all stages of wear cusps which 



* Thomas, P. Z. S. 1908, p. 977. 



