666 



RODENTIA 



anterior border more strongly and evenly rounded, so that 

 triangular outline is less evident ; distribution of enamel essen- 

 tially as in upper tooth. Molars moderately large relatively 

 to size of skull, their enamel pattern well dehned, with sharp 

 salient and re-entrant angles ; upper teeth without noticeable 

 contrasts in length of crowns, but m^ decidedly longer than 

 either of the succeeding mandibular teeth. First upper molar 

 normally with an anterior transverse loop and four alter- 

 nating closed triangles ; rarely a postero-internal loop similar 

 to that in m'^ may be present ;* two re-entrant angles are 

 normally present on each side (a third on inner side when 

 postero-internal loop is present), they are sharply pointed and 

 well defined, their points extending beyond median line of 

 crown, those of outer side slightly narrower and deeper than 

 those of inner side ; anterior loop crescentic in outline, its inner 

 limb longer and narrower than outer limb (though its apex does 

 not extend to level of those of inner triangles), its length 

 decidedly less than greatest width of tooth, its main axis oblique 

 to that of tooth-row ; inner and outer triangles essentially alike 

 in form, those of inner side slightly the larger ; postero-internal 

 loop when fully developed similar to that of m'^, but more often 

 rudimentary. Second upper molar with an anterior transverse 

 loop, two outer and one inner closed triangles, a postero-internal 

 rounded loop about half as large as the largest triangle, and two 

 outer and two inner re-entrant angles ; anterior loop much like 

 that of m^, but wider externally than internally, its long axis 

 nearly transverse to tooth-row, its length nearly equal to greatest 

 Avidth of tooth ; closed triangles similar to last three of preceding 

 tooth ; postero-internal loop oval or elliptical, its long axis 

 directed slightly forward. Third upper molar with anterior loop 

 and three closed triangles essentially like those of m^, except that 

 closed triangles are smaller and transverse loop is nearly 

 symmetrical and about as wide as crown at middle ; behind 

 closed triangles lies a long terminal loop subtended externally 

 by a slight notch and internally by a deep re-entrant angle ; 

 this loop in its simplest form is indented at middle by a deep 

 re-entrant angle which imparts to it a strongly curved crescentic 

 outline, the anterior limb longer and more pointed than the 

 posterior limb ; outer convex surface usually marked by a slight 

 salient angle just behind point of attachment to second outer 

 closed triangle ; in this, the most usual form, the tooth has three 

 well developed re-entrant angles and four well developed salient 

 angles on inner side, two well developed re-entrant angles and a 

 notch and three well developed salient angles and a slight pro- 

 jection on outer side ; in the most complicated form of terminal 



* The presence of this loop is a normal character in the Hebridean 

 form, M. agresiis exsul, some trace of it being visible in ten of the fourteen 

 specimens examined. Among the other races it appears to be uniformly 

 uncommon, occurring in about five per cent, of the skulls seen. 



