928 



half-way along anterior and outer cnr\'ature, its surface smooth, 

 yellowish white. Lower incisor with root extending slightly 

 beyond m.^ ; section of shaft and arrangement of enamel 

 essentially as in upper tooth, but outer border flattened behind 

 enamel plate, thus distorting the semicircular outline. Cheek- 

 teeth agreeing with those of Sciurus vulgaris in general plan of 

 enamel folding but differing in numerous details, principally the 

 result of a tendency toward greater height of tubercles and main 

 ridges. Anterior upper premolar simple, terete, its crown with 

 high, median, obliquely transverse ridge and small anterior 

 and posterior depression, the area of crown about one-half that 

 of succeeding tooth. Upper cheek-teeth with inner tubercle 

 much narrower and higher than in *S^. vulgaris, the length of 

 base scarcely equal to height, the width of base decidedly greater 

 than length ; main ridges (second and fourth) simple but high, 



extending to summit of inner 

 tubercle and forming, together 

 with the tubercle, a narrowly 

 U-shaped figure, the most con- 

 spicuous feature of crown when 

 not worn away ; anterior ridge 

 very narrow, at extreme edge of 

 crown, separated from base of 

 second ridge by a conspicuous 

 furrow, its outer extremity form- 

 ing a slight cusp, its abruptly 

 rounded inner termination lying 

 on anterior surface of base of 

 main tubercle ; third ridge re- 

 presented by a minute cusp 

 between outer extremities of (J- 

 shaped figure, the space between 

 the limbs of the U occupied 

 ]iy a smooth depression ; posterior margin of crown with a ridge 

 terminating internally like anterior ridge, but not extending to 

 outer margin. The principal variations of this pattern are : in 

 pm^, second limb of [J, with its outer cusp somewhat reduced ; 

 in m^, second limb of U obsolete and distorted, not extending to 

 outer border of tooth, the posterior half of crown occupied by a 

 shallow, basin-shaped area usually bearing a low but evident 

 cusp on its inner border. Lower molariform teeth with the 

 same elements as in Sciurus vulgaris, but crowns higher and 

 more compressed, the central depression deeper and narrower, 

 the cusps high and conspicuous, particularly the antero-external 

 and antero-internal, which are connected by a well-developed 

 ridge having much the appearance of a common base to the two 

 cusps ; inner side of bases of two outer cusps joined by a low 

 ridge, beyond which lies a small but rather deep depression. 



Measure luents. — Averaije and extremes of four adults from 



Fig. 189. 

 Cltellus citellus. Cheek-teeth, x 



