TEETH 19 



lingual side of the molars throughout, and its alveolar capsule 

 terminates behind at a point opposite the alveolus of 7n.^. In the 

 Voles the tooth is much longer; it passes backwards on the 

 lingual side of dij and m^, crosses to the labial side of the jaw 



A.B. Bo. 



Fio. 10. — Vertical longitudinal sections of skull of Arvicola amphibius 

 Linna-us (enlarged). 



I. P. interparietal ; Sq. squamosal ; //. optic foramen ; /. olfactory chamber ; 



V. vomer; As. alisphenoid; Me. mesethmoid; B.s. basisplienoid ; 



c.l. transverse canal; P. palatine; P.s. presphenoid; M. maxilla; 



I .a. alveolus of incisor. 

 I'b. turbinals; F. frontal; P. parietal; /. supratympanic fenestra of 



squamosal; P.m. premaxilla ; w^ m*. alveolar capsules; P<. pterygoid ; 



A.B. auditory bulla; Bo. basioccipital. 



between ^Hj and m^, and ascends into the condylar process to a 

 greater or less height, its termination often producing a well- 

 marked hump on the outer surface. 



In structure the incisors are quite typical. Each is developed 

 from a persistent pulj) lodged in the widely open pulp-cavity at its 

 base. The front face of the tooth is formed by a thick plate of 

 hard enamel which thins out and ends off on the lateral surfaces ; 



