20 



MICROTIN^ 



the core of the tooth, exposed on its posterior face, is formed of 

 relatively soft dentine. In transverse section the teeth are usually 

 wider than deep ; the enamel is frequently stained by a yellow 

 pigment; the front face of each upper incisor may or may 



l.af. 



Fio. 11. 



-Skull of Arvicola amphibius Linnseus, with top removed to 

 expose the floor of the braincase. 



P.m. premaxilla; N. nasal; L. lachrymal; M. maxilla; F. frontal; 



•/. jugal; O.s. orbitosphenoid ; P.s. presphenoid; A.s. alisphenoid; 



Sq. squamosal; B.s. basisphenoid ; A.B. periotic; B.o. basioccipital. 

 I.o.f. infraorbital canal; I. olfactory chamber; //. optic foramen; m^, m^. 



alveolar capsules; a.m.i. groove for internal maxillary artery; F.1.2. 



exit for first and second branches of fifth nerve ; F*. foramen ovale ; 



F.l.m. foram,en lacerum medius. 



not be traversed by a longitudinal groove. The teeth differ 

 considerably from genus to genus, and the significance of some of 

 the differences is discussed below. 



The cheek-teeth are highly characteristic. In all Microtinse 

 they are tall-crowned or hypsodont. In some genera they are 

 of limited growth, closing their pulp-cavities and cement-spaces 

 below, developing roots, and wearing out in old age. But in the 



