11 



Fio. 4. — Arvicola amphihixis Linnoous. 



Ventral views of skulls in different stages of growth. Stages IV, V, 

 and VI (same specimens as shown in Fig. 2). 



molars become relatively smaller, the hinder part of the palate is sculptured 

 in bold relief, the pterygoid fossse increase in size, and the bullae develop 

 large eustachian processes. 



