KEY TO GENERA 93 



Cheek-teeth with the inner and outer salient angles 



and infolds approximately equal; re-entrant folds 



with or without cement, not unusually narrow or 



deep; 7«3 with three or four outer and three inner 



salient angles; m^ with five closed triangles 



1. General form of m, " nivaloid." Palate narrower. 



Jixternal characters normal ; fur, feet and pads 



not speciaUy modified; ears moderately large- 



tail long ■' jj° J ' 



3. General form of m, " arValoid.'" Palate 'broader. ™''^' 



b. Skull with temporal ridges widely separaled^'in™^'' 

 interorbital region of adult. 

 1. Lower incisor short, terminating posteriorly in base 

 of condylar process, its shaft not noticeably 

 displacing m,. Palate ^vith postero-median 

 septum but httle inclined, the postero-lateral pits 

 very shallow, and the postero-lateral bridges 

 usuaUy absent. Auditory buUse small ; stapedial 

 artery naked. ^ 



Cheek-teeth characterized by the small size of 

 the outer salient angles of the lower molars; 

 re-entrant folds narrow (giving the teeth a 

 longitudmaUy crowded appearance), not con- 

 taimng cement; enamel thicker on concave 

 than on convex sides of sahent angles; m^ 

 with three salient angles on each side; m, 

 complex with four or five outer and six inner 

 sahent angles, its closed triangles varying in 

 number from three to seven. 

 External characters normal . . . Phenacomys 

 ~. Lower mcisor ascending the condylar process for 

 a greater or less distance, its shaft (at all events 

 in later species) noticeably displacing m,. Palate 

 with postero-median septum, postero-lateral pits 

 and bridges well defined. 



Cheek-teeth with the outer and inner salient 

 angles and re-entrant folds approximately equal- 

 re-entrant folds of normal width and depth 

 usually, but not invariably, containing cement; 

 enamel usually thicker on convex than on con- 

 cave sides of salient angles; m^ with no more 

 than three sahent angles on each side, its second 

 mner infold sometimes persistent, sometimes 

 subject to reduction by insulation of its internal 

 portion; nil never with more than three closed 

 triangles m adult stages of wear, its third outer 

 fold sometimes persistent, but usually reduced 

 by msulation of its internal portion. (In older 

 species the molar roots are developed sooner- 

 m newer species at a later moment in the life of 

 the mdividual ; the islet representing the internal 

 portion of the third outer fold oi m, is a long 

 persistent feature in the earher forms, but 

 becomes quite ephemeral in later species.) 



tMimomys. 



