INTERRELATIONSHIPS OF GENERA 81 



anteriorly they are perforated and placed in rather broad connec- 

 tion with the orbit — a very unusual feature in voles. The auditory 

 bulla} are moderately large, with short tubular external meatus ; 

 the legmen tympani articulates with the squamosal in front and 

 the mastoid portion is not inflated ; the cavity of the bulla does 

 not contain spongy tissue, but its walls are strengthened by 

 several incomplete perpendicular septa; the stapedial artery is 

 not enclosed in a bony tube. The mandible is normal; the 

 lower incisor produces a strong alveolar hump upon the outer 

 surface of the condylar process well below the level of the condyle. 



The upper incisors are strongly curved, their front surfaces 

 flush with the nasal tips and their growing bases lying in the 

 masillaries immediately in front of m^ ; they are broad teeth, 

 each with a weak groove on the outer third of its anterior surface ; 

 the enamel is stained bright yellow. The lower incisors are not 

 peculiar. 



The cheek-teeth are rooted, each tooth with two roots; in 

 m^ a vestige of a third root, supporting the second inner angle, 

 such as occurs in some of the earlier species of Mimomys, is 

 sometimes present ; in m\^ the two roots may coalesce. The 

 enamel is rather thick throughout, but shows feeble traces of a 

 normal differentiation. Cement is not present in the rather wide 

 re-entrant folds. The outer folds both in upper and lower molars 

 tend to leave the opposed dentinal spaces confluent with each other. 

 The enamel pattern in general is like that of Ellobius ; m^, m^, and 

 m^ are essentially normal ; m^ has three outer and two inner salient 

 angles, divided into two lobes by the second outer and single 

 inner infolds ; its outer salient angles are very much reduced, with 

 the first outer triangle very small and broadly confluent with the 

 anterior loop, an exaggeration of the tooth form seen in Alticola 

 and Hyperacrius ; the opposed tooth m^ is also greatly reduced and 

 divided into two lobes, of which the posterior consists of the 

 posterior loop proper and the first inner triangle, which are broadly 

 confluent in consequence of the reduction of the first inner fold; 

 the anterior lobe is formed by the second outer and third inner 

 salient angles, broadly confluent with each other ; the third outer 

 angle is obsolete ; m^ has three outer and four inner salient angles, 

 consisting of a posterior loop, three substantially closed triangles, 

 and an anterior loop formed by the broadly confluent fourth and 

 fifth triangles which open into the short anterior loop proper. 

 Young specimens of the molars show traces of the " intermediate " 

 tubercles and other ephemeral complications (see p. 117). 



Ellobius is widely distributed in Eastern Europe and Asia, 

 and its range, as shown below, appears formerly to have extended 

 into Northern Africa as far west as Tunis and Algeria — the only 

 Microtine known from the African continent. Like Protnetheomys 

 it is highly specialized for fossorial habits ; but its mining opera- 

 tions are conducted in a totally different way, and because the 

 methods employed by the two animals are and have always 



V.L. G 



