ACOMYS 



885 



palate is very short, its length from incisive foramen to the trans- 

 verse ridge homologous with the normal posterior border scarcely 

 exceeding breadth at middle of first molar. Hamulars 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 bullae moder- 

 ately large but not specially 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 Epimys raltus or 

 Mus musculus). Plate forming outer wall of infraorbital foramen 

 broad, rather squarely rounded off above. Infraorbital foramen 

 rather widely open 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 root of lower incisor slight. 



Teeth. — Upper incisor strongly compressed, rather more so 

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

 essentially like the upper tooth 

 in cross section, its anterior and 

 posterior diameters nearly equal. 

 Molars suggesting those of Mus 

 musculus both in relative size and 

 in arrangement of tubercles, but 

 the general tendency toward 

 reduction carried less far, so 

 that crown area of m l is about 

 equal to that of m 2 and m 3 

 combined. First upper molar 

 with crown relatively wider 

 than in Mus musculus, but 

 with il similarly though less 

 displaced backward. Second 

 upper molar with t3 represented 

 by a rather evident antero- 

 external shelf, usually bearing 

 a minute tubercle ; t% relatively 

 better developed than in the house mouse. Third upper molar 

 with crown area equal to fully one-half that of m 2 ; tZ usually 

 represented by a minute antero-external tubercle ; second and 

 third laminse 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 laminse obsolete.* Lower molars as m 



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

 upper molars are sufficiently worn to obscure the pattern, particularly of m 3 . 



Fia. 181. 



Acomys rainous. Cheek-teeth, 



somewhat worn. X 10. 



