146 MICROTIN^ 



spine. Pterygoid fossae deep, their floors distinctly dorsal to the 

 ventral surface of the basisphenoid. Auditory bullae of small 

 or medium size ; middle ear filled with a rather dense sponge 

 of bone ; canaliculus tympanicus completely ossified ; tegmen 

 tympani articulating as iisual with the squamosal ; mastoid 

 portion slightly but noticeably inflated. Basioccipital moderately 

 broad in front. 



In consequence of the shortness and wholly lingual coiirse of 

 the lower incisors, the mandible, like that of other lemmings, 

 differs from that of the voles in the stoutness of its horizontal, 

 and slenderness of its ascending rami. In adults the lower 

 incisor on each side terminates near the hinder edge of m^; but 

 in the newborn it does not pass m^. In each ramus the anterior 

 border of the coronoid process rises above the molar level at a 

 point opposite the middle of m^. Angular processes large and of 

 normal form. 



Dentition. — Upper incisors without grooves, and with normal 

 cutting edges. Lower incisors short, traversing the lower jaw 

 on the lingual side of the molars to terminate opposite m.g. 



Cheek-teeth rootless and persistently growing, with their re- 

 entrant folds destitute of cement. When unworn with tubercular 

 caps; when worn displaying a normal prismatic pattern. In 

 adult stages of wear (Figs. 71, 72) all the triangles are sub- 

 stantiaUy closed, of somewhat peculiar, transversely elongated 

 form, the inner and outer salient angles being subequal in size 

 in each tooth. Enamel conspicuously differentiated into thick 

 and thin portions, forming respectively the concave and convex 

 sides of the salient angles as in the higher members of the genus 

 Microtus, becoming very thin at, or lacking altogether from, 

 the tips of the sahent angles. The patterns of the upper teeth 

 (Fig. 71) are as follows : — m^ with an anterior loop, foUowed 

 by six alternating triangles, of which the postero-external one 

 is much reduced, and with four salient angles on each side ; m^ 

 with an anterior loop and five triangles (three external, two 

 internal), of which the postero-external one is reduced, and with 

 four outer and three inner salient angles ; m^ with an anterior 

 loop, four closed triangles, a posterior loop formed by two 

 confluent and more or less reduced triangles, and with four 

 salient angles on each side. In most of the known species a 

 vestigial fifth inner angle is developed at the hinder end of m^ 

 and a similar vestigial fourth inner angle at the hinder end of 

 m^ ; but in two species, the living D. hudsonius and the extinct 

 D. henseli, these vestiges are absent (cf. Figs. 71, 4 and 71, 7) and 

 the posterior wall of the triangle immediately in front (fourth 

 inner angle in m^, third inner in m^) is reduced, losing its concave 

 form and a great deal of its thick enamel. This distinction, first 

 pointed out by HenseP in 1858 and Forsyth Major 2 in 1872, 



^ Hensel, Zeitschr. deutsch geol. Gesellsch., 8, p. 280. 

 ^ Forsyth Major, Atti Soc. Ital. Sc. Nat., 15, p. 125. 



