﻿July, 1896.] 



SUBGENUS MICROTUS. 



63 



and others, Witliin certain limits, liowever, tlie skull varies consider- 

 ably in size and form, so that it is difficult to frame any accurate diag- 

 nosis. The skull of Microtus arvalis figured on Plate I represents the 

 form characteristic of the great majority of species. One of the most 

 notable departures from this type is seen in the skull of Microtus nivalis^ 

 which has an unusually low, broad brain case, and liat dorsal outline. 



Bony palate. — The bony palate in the subgenus Microtus (fig. 7 A, and 

 PL IT, fig. 5) shows in its most perfect development the form which may 

 be considered the normal one in the genus, since it is characteristic of 

 most of the subgenera and of the vast majority of species. As this 

 palate has already been described (px). 26-27) it is necessary here to notice 

 a few departures from the type form only. In young individuals the 

 sloping ridge is broader than in the adults, while in very old individuals 

 it often becomes very abrupt and at the same time greatly narrowed. 

 These two extremes, which are usually characteristic of immaturity and 

 old age, occur as the normal condition in the adults of certain species. 

 In Microtus nivalis the ridge is broad and fiat, while in M. agrestis^ M, 

 ratticeps, and most of 



''Mm 



the American species 

 it is narrow and ab- 

 rui^t. Occasionally (es- 

 pecially in 31, agrestis 

 and M. ratticeps) the 

 anterior edge of the 

 iilterpterygoid fossa is 

 encroached ui)on by 

 the projecting median 

 ridge. The latter, on 

 the other hand, may be slightly cleft in the median line, thus fore- 

 shadowing the first step in the series of changes which lead to the very 

 different palate of Evotomys. 



Enamel pattern in general. — The enamel pattern in the subgenus 

 Microtus (fig. 33) is characterized by the large number of loops and 

 angles in the first lower molar and last upper molar. 



Front lower molar. — The first lower molar normally contains a pos- 

 terior transverse loop, five closed triangles, two of which are on the outer 

 side and three on the inner side, and finally an anterior loop which is 

 usually more or less deeply cut by two reentrant angles, one on each 

 side of the loox), the outer of which is always the more posterior of the 

 two. With these loops and triangles are usually associated nine well- 

 developed salient angles, two formed by the i)osterior transverse loop, 

 one by each of the five closed triangles, and one by each side of the 

 base of the anterior loop. That part of the anterior loop which lies in 

 front of the reentrant angles may develop a salient angle on its inner 

 side, less frequently one on the outer side. Very rarely the loop may 

 be cut by a third reentrant angle. This condition occurs in adult spec- 



EiG. 33.— Enamel pattprn of 

 arvalis ; (b) J/. (J/.) vlvali 

 {21.) ratticeps. (x5.) 



molar t<>ctli, (a) inrrofif. 

 : ((•) J/, (^i.) pcnnsijlvanl 



(iricrotiis) 

 IS: {d)M.- 



