INTERRELATIONSHIPS OF GENERA 43 



tions, are the members of the E. rufocanus group. These are 

 specialized for subsistence upon a coarser and tougher diet than 

 that used by the glareolus group. The general size of the animal 

 is increased. The cheek-teeth have become taller-crowned and 

 more robust ; the alveolar capsules of irfi and m^ rise up promi- 

 nently in the sphenorbital fissure, subsiding only in old age, 

 after the failure of the dentinal pulps and the formation of roots 

 to the teeth ; m^ is encapsulated and noticeably displaced by the 

 shaft of the lower incisor. In pattern the teeth are somewhat 

 modified ; the inner and outer salient angles acquire a perfect 

 alternation and in middle stages of wear the dentinal spaces are 

 tightly closed, although they are confluent in young stages of 

 wear and again acquire an " Evotomys-l^ke " confiuency in extreme 

 ojjl age ; m^ is reduced in adults and has only three salient angles 

 on each side ; but when young both it and m-^ show commonly 

 ephemeral complications. One character in which the members 

 of this group are rather more primitive than are most of the other 

 species of Evotomys may be mentioned; m^ and m^ in young 

 stages of wear frequently show traces of cusp n. In correlation 

 with the larger and heavier molars the temporal muscles have 

 become stronger, and under their combined influence the skull 

 has acquired a massiveness and angularity which resembles that 

 seen in many species of Microtus. In the interorbital region, 

 which becomes more constricted, the temporal ridges approach 

 each other with age and in extreme old age may even come in 

 contact, although usually they remain separated by a deep and 

 narrow sulcus. 



AscHizoMYS, known from only one individual, is certainly a 

 remarkable member of this group in which the outward form has 

 become lemming-like. The skull is described as being essentially 

 as in normal Evotomys, broad, depressed, lightly built, smooth and 

 rounded. The cheek-teeth are said to be persistently growing, 

 but with the general pattern and rounded salient angles so 

 characteristic of less specialized Evotomys ; m^ is small and weak 

 with a well-developed fourth outer angle; in the mandible 

 njg is encapsulated and displaced by the incisor as in the E. rufo- 

 canus group. These cranial and dental characters, coupled with 

 my own experience of the younger stages of growth of E. rufo- 

 canus, lead me, however, as explained below, to suspect that 

 Aschizomys is not of generic value, but that it is based upon an 

 adolescent specimen of a more or less aberrant member of the 

 E. rufocanus group. 



EoTHENOMYS is apparently descended from some primitive 

 form of Evotoinys. Externally the only important modification 

 is seen in the mammary formula which is reduced from the 

 normal 2 — 2 = 8 to — 2 = 4. In all essential respects the 

 skull resembles that of Evotomys. The cheek-teeth have become 

 completely hypsodont and rootless, and in the lower jaw m^ 

 is noticeably displaced by the incisor. In one respect the molars 



