24 



MICROTIN^ 



closing the jaws, and its most anterior fibres pull the lower jaw 

 forwards as well as upwards. 



Temporalis. This muscle although varying considerably 

 in size and in its precise relation to the braincase in Microtinse 



M. M. a. 



Fig. 13. — Arvicola amphibius Linnaeus. 



Jaw muscles : M.L.a. anterior or superficial portion of masseter lateralis ; 

 M.L.p. posterior or deep j)ortion of masseter lateralis; M.M.a. and 

 M.M.p. anterior and posterior portions of masseter medialis; T. 

 temporalis. 



is always well developed. The temporal fossa extends from the 

 occipital crest to a point more or less far forwards in the inter- 

 orbital region. The muscle is covered by a fascia which extends 

 from the temporal ridge, marking its upper limit of origin, to the 



