885 



palate is very short, its leiif^tli from incisive foramen to the trans- 

 Terse i-i(lge liomologous with the noi-nial posterioi- bm'der scarcely 

 exceedini^ breadth at middle of tirst molar. Haniulars diverging 

 posteriorly, their tips broadly in contact with well developed 

 beak of auditory bulla, the greatest distance between them about 

 equal to length of interpterygoid space. Auditory buUfe moder- 

 ately large but not sj^ecially inflated, their form more elongated 

 transversely to main axis of skull than in members of the other 

 European genera. Zygomata strong, somewhat expanded 

 upward anteriorly (conspicuously more than in Ephnys rattus or 

 Mas musculiis). Plate forming outer wall of infraorbital foramen 

 broad, rather stjuarely rounded oft" above. Infraorbital foramen 

 rather widely oj-ien below owing to the unusually slight pro- 

 tuberance formed by the root of upper incisor. Mandible with 

 long, rather narrow and straight angular process, the lower 

 border of which is folded squarely inward ; coronoid process 

 reduced to a mere recurved spicule scarcely rising above level of 

 wide, nearly horizontal area extending from its base to articular 

 surface. Protuberance over I'oot of lower incisor slight. 



Teeth. — Upper incisor strongly compressed, I'ather more so 

 than in Mus musculus, the cutting edge normal. Lower incisor 

 essentially like the iqiper tooth 

 in cross section, its anterior and 

 posterior diameters nearly equal. 

 Molars suggesting those of Mus 

 7nmculus both in relative size and 

 in arrangement of tubercles, but 

 the general tendency toward 

 reduction carried less far, so 

 that crown area of m^ is about 

 equal to that of m'^ and m'^ 

 combined. First upper molar 

 with crown relatively wider 

 than in 3Ius musculus, but 

 with <1 similarly though less 

 displaced backward. Second 

 upper molar with /3 represented 

 by a rather evident antero- 

 external shelf, usually bearing 

 a minute tubercle ; /9 relatively 



better developed than in the house mouse. Third upper molar 

 with crown area equal to fully one-half that of irr ; t'i usually 

 represented by a minute antero-external tubercle ; second and 

 third laminae rather distinct, oblique, separated by a narrow 

 groove, the extremities of which produce a slight re-entrant 

 angle on each margin of crown, the tubercular elements entering 

 into formation of the laminw obsolete.* Lower molars as in 



* In the specimen from which fig. 181 was drawn the crowns of the 

 upper molars are sufficiently worn to obscure the pattern, particularly of ?H^ 



Fig. 181. 



Acomns minou!'. Clieek-teeth, 



somewhat worn, x 10. 



