THE TEETH OE MAMMALS 139 



The premolars are small, of triangular form, and 

 implanted by three roots. The upper true molars 

 are large, and are marked by two transverse 

 crests. The lower molars have a central depres- 

 sion with a marginal ridge and with cusps at each 

 corner (Figs. 48 and 49). The Porcupine's teeth 

 are similar to the Squirrel's, except that the 

 molars have three transverse ridges inclosing two 



FlG. 49. — Roentgenogram of skull of Squirrel, showing growth of 

 incisor. (By Dr. E. H. Skinner.) 



valleys. In some of this family these crests and 

 valleys are highly complicated. The common Eat 

 has the formula, — 



. 1-1 0-0 0-0 3-3 * a 



l. -— > c. ■—- p.m. m. — — = 16. 



1-1 0-0 F 0-0 3-3 



Deciduous teeth are entirely wanting in this fam- 

 ily, so that it is monophyodont. The molars de- 

 crease in size from front to back, are simply 

 tuberculate, and implanted by three roots. The 

 tubercles are often worn away in aged individuals. 



