MICROTINjE 



Fig. 1. — Arvicola amphibins Linnscus. 



Dorsal views of skulls in different stages of growth. Condylo-basal length, 

 Stage I, 25-1 mm., St. II, 31-2 mm., St. Ill, 35-5 mm. 



Figs. I and 2 illustrate the changes, which accompany growth, in the 

 relative proportions of the cranial and facial parts of the skull, the progres- 

 sive constriction of the interorbital region, gradual development, approxi- 

 mation and fusion of the temporal ridges, increasing salience of the post- 



