sciURUs 903 



by a single low, elongated tubercle, its height much less than 

 length of base, the width of base scarcely more than half its 

 length, the median portion concave transversely, the concavities 

 of the four teeth forming together a longitudinal furrow, the 

 outer side, except of m'\ with four low cusps from which trans- 

 Averse ridges extend inward on or across crown, their general 

 direction somewhat obliquely forward. The usual scheme of 

 the outer cusps and their ridges is as follows : first cusp low, 

 its ridge forming anterior margin of crown and extending to 

 inner extremity of base of inner tubercle ; second cusp nearly 

 twice the height of first, its ridge extending to side of base of 

 inner tubercle ; third cusp like first, its ridge extending only 

 about half way across crown ; fourth cusp like second, its ridge 

 extending to slightly behind middle of base of inner tubercle ; 

 posterior base of inner tubercle continued outward along hinder 



Fia. 1S7. 

 Sciiirtif! vulgaris. Cheek-teeth, x 



border of crown as an ill-defined ridge extending nearly to 

 base of fourth outer cusp. The principal deviations from this 

 pattern are : in pm*, first outer cusp nearly as large as second, 

 fourth transverse ridge with an evident tubercle near its inner 

 extremity ; vv^ all the outer cusps together with their transverse 

 ridges obsolete except the second, the posterior two-thirds of 

 crown occupied by a shallow basin-like depression. Lower pre- 

 molar with three roots, lower molars with four. Crowns squarish 

 in outline, that of m^ longer than broad, the others slightly 

 broader than long, that of j'^'h with antero-external border so 

 rounded off as to give the ci'own a somewhat triangular outline. 

 Main portion of crown occupied Iw a smooth depression, on both 

 outer and inner edge of which are thrcio low tubercles, the 

 anterior and posterior outer ha\ing the broadest and best 

 defined bases, while the anterior internal is the highest ; on each 



