120 MICROTIN^ 



and corresponding outer cusps of the lower molars are reduced to 

 two (6 and 7). Cusps 4, 5, 6, and 7 have become dominant and 

 the teeth make a close approach to that form, with two large 

 cusps (6 and 7) on one side and the remains of five cusps separated 

 by three enamel islands or folds on the other, which is seen in so 

 many rodents in all the three great tribes of Simplicidentata. 

 Cricetodon has thus specialized in a direction quite different from 

 that followed by the Microtinae ; although some of its dental 

 characters point back to the same common source, it cannot be 

 considered as in any way ancestral to the group with which we are 

 dealing. 



The Nesomyinse now restricted to Madagascar are of especial 

 interest, inasmuch as one of the genera, Brachytarsomys, has 

 developed in such a way that Forsyth Major was able to describe 

 it as " a forerunner of the Microtinae." ^ The most primitive 

 genus is Nesomys. Its cheek-teeth are brachyodont, rooted, 

 and complex. Like those of Cricetodon, they show cusps 

 1, 4, 2, 5, and 3 well developed, with a trace of x^ in in}, and 

 cusps 6 and 7 are the chief elements on the opposite sides of the 

 teeth. But all the cusps are about equally developed and, what 

 is more important, the median row is well represented, not 

 specially reduced as in Cricetodon. In this one respect Nesomys 

 makes a nearer approach to the voles. The vole-like Brachy- 

 tarsomys is generally regarded as a fossorial modification of the 

 Nesomyinae in which the cheek-teeth (Fig. 63) have acquired a 

 strikingly Microtine general appearance, although they are far 

 more brachyodont than in any known vole. The molars agree 

 further with those of Microtinae in lacking any trace of an 

 independent cusp 2. The skull (Fig. 18) under the influence of 

 fossorial habits and of jaw muscles, which have developed exactly 

 as in some of the higher voles, has become almost the counterpart 

 of that of ArvicoJa or Microtus in the advanced position of the 

 orbit, the structure of the infraorbital canal, the zygomatic arch, 

 the form and course of the temporal ridges (which fuse in front 

 to form an interorbital crest), and the flattening of the braincase. 

 In other respects, correlated chiefly with the extreme brachyo- 

 donty, the skull of Brachytarsomys is very primitive. Although 

 in both respects modified in uiuch the same manner as in the higher 

 voles, the cheek-teeth are already too reduced and the skull is too 

 highly specialized for the genus to be considered as representing 

 the ancestor common to all Microtinae ; but that it has 

 descended from that ancestor there can be little doubt and it may 

 perhaps be necessary later on to transfer Brachytarsomys from the 

 Nesomyinae to the Microtinae. Pending the examination of spirit 

 material and unworn teeth it is not advisable to make such a 

 change. 



Probably on account of the close superficial resemblance which 

 exists between the adult cheek-teeth of Microtinae and those of 

 1 Forsyth Major, Proc. Zool. Soc, 1897, p. 719. 



