﻿July, 1896.] 



SUBGENUS ARVICOLA. 



67 



(PI. I, fig. 1) it is smaller, tliougli cousiclerabl}^ larger than in most 

 species of j17 /cro/zis proper. Aside from its large size and prominent 

 ridges, tlie skull of ArvicoJa differs from tliat of Microtus in its broader, 

 shorter brain case, more widely flaring zj^gomatic arches, and propor- 

 tionally slender rostrum. The peculiar appearance of the rostrum is 

 heightened by the fact that the incisors project more than usual. Some 

 of these characters are more noticeable in the American species, though 

 the latter show no cranial x^eculiarities of sufficient importance to sep- 

 arate them subgenerically from those of the Old World. In the Amer- 

 ican species the skull is usually more lightly built and less strongly 

 angular than in the tyi:>ical members of the genus (compare figs. 1 and 

 9 of ri. I). 



Bony palate. — The bony palate is usually normal, but oc<^^'asionally 

 the median sloping ridge is divided in the median line, so that the 

 interpterygoid fossa is hastate anteriorly (PI. Ill, fig. 7). This condi- 

 tion occurs most frequently in the Ameri- 

 can species, but even among these it is 

 inconstant. 



Enamel J) att em in general. — The enamel 

 pattern in typicalJLrr/eo/rt (fig. 34/^) is char- 

 acterized by the great reduction in the 

 number of closed triangles and salient 

 angles in the front lower molar and 

 back upper molar. In these peculiarities, 

 though closely approached b}^ Fifymys, 

 Pedomys, and PhaiomySj it presents the 

 extreme conditions found in the genus. 

 The third lower molar shows the tendencj^ 

 to closure of the lateral triangles charac- 

 teristic of all the larger members of the genus. The pattern of enamel 

 folding in the molar teeth of the American s\)ec\es of Arvi col a (fig. 31^) 

 is, on the other hand, exactly like that of the tetramerodont species of 

 the subgenus Microtus (e. g., Microius arvalis and most of the western 

 American species). 



Front loiver molar. — In the typical species the simplification in the 

 structure of the teeth is carried furthest in the first lower molar. This 

 tooth normally contains a posterior transverse loop followed by three 

 closed triangles (one on the outer side, two on the inner side) and a 

 terminal transverse loop which is deeply constricted in the middle. 

 Each transverse loop forms two salient angles and each lateral triangle 

 one, making seven in all. Deviations from this form are very rare. In 

 one or two specimens I have seen a fourth triangle isolated on the outer 

 side, thus producing a tooth much like the corresponding one in Micro- 

 U(s {2Ik'roius) raiticeps^ a species which has the last upper molar very 

 complicated in structure. The front lower molar in tj^pical Arvicola 

 differs from that of the other groups in which it has only three closed 

 triangles in the reduced number of salient angles — seven instead of 



Fig. 34.— Enamel pattern of molar teeth, 



(a) Microtua (Arvicola) inacropus ; 



(b) M. (A.) terrestris. (x5.) 



