166 MICROTINiE 



portions of the temporal fossae ; supratympanic fenestrse relatively 

 large ; owing to the slighter lateral saliency of the lambdoid 

 crest, the supratympanic portions of the temporal fossae are less 

 extensive transversely than in Leinmus. Interparietal rather 

 longer in relation to its width, truncated laterally (although 

 not crossed by the temporal ridges), and usually with a boldly 

 convex posterior border. In the ventral view the great width 

 and shortness of the rostrum are consjiicuous. Antero-palatal 

 foramina rather short but unusually broad. Cheek-teeth narrower 

 in proportion to their length, the tooth rows, about equal to the 

 diastema in length, diverging posteriorly, though not so rapidly 

 as in Leninius. Palate posteriorly nearly as in Microtus ; the 

 posterior median sloping septum longer and better developed 

 than in Leminus, the inner or mesial borders of the post-palatal 

 pits not extending so far forwards but bending inwards to make 

 a junction with the sides of the median septum whereby an 

 appearance, closely similar to that seen in many species of 

 Microtus, is imparted to the hinder edge of the palate. Ptery- 

 goid fossae slightly longer but a little shallower than in Lemmus, 

 their floors being scarcely dorsal to the ventral surface of the 

 basisphenoid. Choanse, presphenoid, basisphenoid, and fore-part 

 of basioccipital all considerably narrower than in Leuivtus. 

 Auditory bullae relatively large and globular, more closely ap- 

 proximated to the middle line below, their inner sides being 

 conspicuously more inflated than in Lemmus ; in each bulla the 

 external meatus is very shortly tubular; the walls are densely 

 spongy, the sponge being connected with the surface of the 

 petrous portion by a wide mesh work of strong bony trabeculae; 

 canaliculus tympanicus ossified until it reaches the staj^es ; 

 mastoid portion and tegmen tympani less inflated than in 

 Le>iimus. 



Mandible essentially as in Lemmus, but condylar jjrocess in 

 each ramus shorter and the groove between the cheek-teeth and 

 the ascending ramus more evenly and smoothly continuous with 

 the concave inner surface of the angular process behind. 



Dentition,. — Incisors like those of Lemmus as regards their 

 courses and lengths. Ujiper incisors strongly curved, " opistho- 

 dont," each with a longitudinal groove near the outer edge of 

 its anterior surface ; cutting edges of upjjer incisors as in Lem- 

 mus, when uninjured, the soft dentine worn back and hollowed 

 considerably, the hard and resistant anterior enamel jilate 

 becoming very salient and forming a tubular or gouge-like 

 termination. 



Cheek-teeth rootless ; infolds filled with cement ; enamel well 

 diiJerentiated into thick and thin jiortions forming respectively 

 the concave and the convex sides of the salient angles. Enamel 

 pattern essentially as in Lemmus ; but in some species jiersistent 

 though quite vestigial traces of the median row of tubercles 

 occur frequently (Fig. 73). In m^ the second and third transverse 



