586 



RODENTIA 



usually subterete or a little flattened against succeeding tooth, 

 its area about one-fifth that of first molar ; cross ridges two or 

 sometimes three, the third smaller than the others and lying at 

 back of upper tooth and front of lower tooth, the two main 

 ridges of upper premolar usually joined at inner side, and 

 occasionally at outer side also, producing a circular, raised rim. 

 Milk premolar minute, not functional, its crown bluntly spicular, 

 smooth, scarcely one-quarter as large as that of permanent tooth. 

 First upper molar about one-half greater than that of second, its 

 length decidedly greater than its width, the anterior border 

 oblique, not so long as posterior border, the inner border with 

 a slight though abrupt emai'gination anteriorly ; surface crossed 

 by five high, very distinct ridges, the first three wider apart than 

 the last two and sloping obliquely backward toward inner 

 margin, the first independent of the others, the second, third, 



fourth, and fifth continuous with 

 a ridge which extends along inner 

 border as far forward as space 

 separating first ridge from the 

 others ; when viewed from the 

 side the outer terminations of the 

 five ridges have the appearance of 

 five low cusps, decreasing regularly 

 in height from before backward ; 

 on inner margin there is a single 

 anterior cusp, the rest of the crown 

 appearing flat. In some specimens 

 there is a rudimentary ridge at 

 extreme anterior border of tooth 

 and traces of another in the space 

 between third and fourth main 

 ridges. Second upper molar 

 squarish in outline, its length 

 slightly greater than its width ; 

 surface of crown crossed by seven low but complete, squarely 

 transverse, equally spaced ridges, all* extending from border to 

 border, the outer extremities appearing as five or six low cusps. 

 Third upper molar like second, but smaller and with posterior 

 border rounded, the ridges behind the first three or four 

 usually incomplete and confused. Lower molars essentially 

 alike in structure, the crown of each tooth crossed by six 

 ridges rather more distinct than those of iv? (except in m^), 

 their terminations producing the usual effect of low cusps along 

 outer border ; crown area diminishing regularly from m-^ to m^ 

 (which is only a little more than half that of anterior 

 tooth) ; iHo and m^ squarish in outline, slightly longer than wide ; 

 TOj decidedly longer than wide, narrower anteriorly than 

 posteriorly. 



* Except occasionally the third. 



Fig. 116. 



Muscardinus aveUanariiis. 

 Cheek-teeth, x 10. 



