EVOTOMYS 245 



hairs, the annulations usually more or less visible ; provided with 

 a thin terminal pencil, and more or less distinctly bicoloured. 

 Manimge, 2 — 2 = 8. 



Skull large and massive, resembling in these respects that of 

 E. cccsariiis, but more angular and " Microtus-like " than in 

 that species. Zygomatic arches moderately divergent, but very 

 light. Antorbital foramen unusually narrow. Rostrum and 

 interorbital region rather long. Interorbital region moderately 

 constricted, parallel-sided in adults; the superciliary ridges weak 

 and widely separated in young adults, becoming stronger and 

 more closely approximated in old age. Post-orbital squamosal 

 crests large, sometimes tending to assume a peg-like form con- 

 spicuous in the dorsal view, sometimes larger, though less con- 

 spicuous, forming the entire edges of the shoulders of the brain- 

 case. Temporal ridges on braincase clearly defined, though 

 not very salient. Braincase large and rectangular, the occiput 

 more squarely truncated than usual in Evotomys. Palate usually 

 narrow, but normal in structure; its posterior margin, however, 

 often with a notched median projection. Choanse narrow. 

 Pterygoid fossae rather short but deep, their floors distinctly 

 dorsal (in adults) to the ventral surface of the basisjihenoid. 

 Auditory bullas moderately large. Mandible normal; but rn^ 

 (robust and hyjjsodont like the other molars) more obviously 

 displaced lingually by the shaft of the lower incisor, and more 

 encapsuled than usual in Evotomys — a character giving the 

 mandible a strong resemblance to that of Microtus, etc. 



Cheek-teeth unusually large, heavy and tall-crowned, charac- 

 terized by more perfect alternation of the inner and outer salient 

 angles, by deeper infolds, and by tighter closure of the dentinal 

 spaces than in other species of the genus ; in these respects 

 E. riifocanus makes a decided ajaproach towards the condition 

 found in Microtus and other higher genera. But, as in all other 

 species of Evotomys, the cheek-teeth are of limited growth, for 

 their enamel organs and dentinal pulj^s fail as old age advances, 

 and fangs are then developed in the usual manner. While the 

 teeth are growing, and for some considerable time after they have 

 acquired roots, their hypsodont character is very noticeable, as 

 their implanted jjortions occupy great cajDsules, which in the 

 upper jaw rise considerably above the general level of the corpus 

 tnaxillaris ; the capsule of m^ is particularly conspicuous, as 

 it forms a great mound blocking the mouth of the sphenorbital 

 fissure, the outer surface of the mound being channelled antero- 

 posteriorly by the infraorbital branch of the internal maxillary 

 artery ; but as age advances and as the base of each molar crown 

 is gradually extruded from its alveolus the capsules subside and 

 eventually disappear. In essential pattern the teeth agree with 

 those of normal Evotomys ; m^ is, however, in adults noticeably 

 simplified, its posterior loop being shortened, its triangles tightly 

 closed, and there being then only three salient angles on each 



