as Enemies of Mankind. 



49 



is performed, are highly modified. Each has a thick plate of hard 

 enamel on its anterior surface, but the relatively soft dentine is 

 exposed for a greater or less width on the posterior side ; conse- 



A B 



Fig, 2. — Field Vole [Microtus agrestis), skull enlarged. 



A. Dorsal, B. ventral, C. lateral views ; p.m. premaxilla, m. maxilla, 

 j. jugal, s. sc[uainosal, p. parietal, t.r. left temporal ridge ; arrows in A. 

 and C. pass through infraorbital canal ; dotted lines in C. indicate 

 course of upper incisor. D. right ramus of lower jaw dissected, i. the 

 incisor, a. its anterior enamel, p. c. its pulp-cavity, c.t. the rootless molars. 



quently with w^ear the tooth develops and maintains a chisel-like 

 cutting edge. The shaft of each upper incisor forms a large 

 segment of a small circle and passes back through the premaxilla 

 into the maxilla, where it terminates near the anterior molar ; 



