60 



MICROTIN^ 



structure of the skull, and longer tail, Orthriomys remains nearer 

 to Phenacomys than does Herpetomys, which in the features 

 mentioned makes a nearer approach to Microtus. In both 

 genera the number of closed triangles in front of the posterior 

 loop in m^ varies between three and five, and in this they agree 

 with Phenacomys, in which m^ displays a similar though wider 

 (3-7) variability; elsewhere in the group we find that the posses- 

 sion of only three closed triangles by m^ is a most constant 

 generic character of ancient standing dating, in the case of Pitymys, 

 at least from the Pliocene period, and that variability in the 

 number of closed triangles in m^ is shown only by those genera 

 in which less than four never occur. In the atrophy or complete 

 suppression of the inguinal mammae, Orthriomys and Herpetomys 

 differ rather strikingly from other members of the subfamily, 

 in which usually the inguinal pairs are constant and the pectoral 

 pairs are liable to reduction. The flattening of the skull in 



Tig. 33. — Cheek-teeth of Proedromys Thomas. 

 Crown views : a. left upper, 6. right lower molars. 



Orthriomys is, no doubt, as in the genus Pitymys, a specialization 

 correlated with its more fossorial habits. 



Of the genera constituting the Microtus-gxovi^, Proedromys, 

 known from a single specimen collected in Western China, is in 

 two respects the most primitive. Its upper incisors are very 

 broad, recurved, and grooved, and its lower incisors are remark- 

 ably short, scarcely invading the condylar process behind. But 

 in the other characters of the skull and teeth it is quite highly 

 speciaUzed. The cheek-teeth are rootless, tall-crowned, and 

 broad, with normally differentiated enamel, and with cement 

 in the re-entrant folds. The enamel pattern is generally as in 

 Microtus, but m^, m^, and 3H3 are somewhat more reduced than 

 usual; m^ has only three outer and two inner salient angles, its 

 posterior loop formed mostly by the third outer angle; m^ 

 has only four closed triangles, the fifth triangle or fourth inner 

 angle being broadly confluent with the short, rounded anterior 

 loop; in m^ the third outer angle is obsolete. The skull is 

 massively built. The temporal ridges probably fuse in extreme 



