630 



RODENTIA 



noticeable ; m^ nearly equal to m.-, and in.f combined, m.^ slightly 

 shorter than m^. First upper molar with a transverse, evenly 

 and broadly crescentic anterior loop and four alternating, 

 approximately equal closed triangles, those on outer side slightly 

 the larger ; each side with three salient and two re-entrant 

 angles. In young individuals the outer re-entrant angles are 

 conspicuously longer than those on inner side, but much more 

 oblique in direction, so that they do not extend appreciably 

 further beyond middle of crown ; the outer salient folds are at 

 this stage rather acute, and the inner folds though blunt are 

 distinctly angular. With increasing age first the inner folds 

 and later the outer folds lose their angularity and assume an 

 indefinite rounded or semicircular form, while the discrepancy 

 in depth between the outer and inner re-entrant angles becomes 

 less marked. Second upper molar with anterior transverse loop, 

 one inner and two outer closed triangles ; outer side with two 

 re-entrant and three salient angles, inner side with one re-entrant 

 and two salient angles. Anterior loop unsymmetrical, broadly 

 rounded internally, narrowly pointed and slightly curved back- 

 ward externally ; rest of crown essentially like last three prisms 

 of m^, but discrepancies in form between outer and inner 

 elements less mai'ked in youth and earlier disappearing with age. 

 Third upper molar with antei'ior transverse loop similar to that 

 of m- but with outer, narrowed portion relatively less developed ; 

 following this are an inner and two outer triangles, closed or 

 variously opened, and a posterior longitudinal loop of variable 

 form ; inner triangle the largest of the three, though usually 

 not much exceeding antero-external triangle ; posteroexternal 

 triangle decidedly smaller than either of the others ; all three 

 triangles may be completely isolated from each other and from 

 terminal loops, or all may be open at both sides, leaving a 

 continuous dentine area along middle of crown ; usually the 

 enamel of the two sides comes in contact at one or more points, 

 and in the majority of specimens the inner triangle is completely 

 or essentially closed. The terminal loop when at its smallest is 

 short and perfectly simple in form, its greatest diameter oblique 

 to the tooth-row and equal to a little more than half that of 

 anterior loop ; when at its largest its greatest diameter distinctly 

 exceeds that of anterior loop, a shallow but evident re-entrant 

 angle is developed at its antero-external base just behind second 

 outer closed triangle, and its inner side is cut by a deep 

 re-entrant angle extending across to enamel of outer side.* 

 In its most simple form the tooth has three salient and 

 two re-enti'ant angles on each side ; in its most complicated 

 form it has three salient and three re-entrant angles on outer 

 side, four salient and four re-entrant angles on inner side. First 



* This angle is best developed cluriiig tlie earlier and middle stages of 

 crowu-wear ; later it tends to become obsolete at a less advanced stage 

 than those forming part of the strictly normal pattern. The frequency of 



