SKULL 



15 



encroaching upon the frontals to an unusual extent. In the 

 post-glenoid region there is typically a large fenestration in each 

 squamosal immediately above the auditory bulla; but. in some 

 genera the fenestra is represented merely by a small foramen. 



The interparietal is usually large. In genera in which the 

 tem})oral ridges sweep by and do not cross the lateral extremities 

 of the bone its form is to a very large extent influenced by the 

 degree in which those ridges are approximated posteriorly; 

 as the ridges tend to become closer together with age, the inter- 

 parietal becomes narrower in relation to its length. In genera in 

 which the ridges traverse the interparietal that bone becomes 

 very small, as in Ondatra and Prometheomys ; in the latter genus 



TEHaO 



Fig. 8. — Arvicola arwpliibius Linnaeus. 



Anterior and jjosterior views of skull with mandible in position : s.r. upper 

 maxillary root of the zygoma ; i.r. lower maxillary root of the zygoma, 

 forming the outer wall of the infraorbital canal or " masseteric plate " ; 

 m. mastoid portion of periotic. 



the sagittal suture persists, even in old age, dividing the bone 

 into two halves, a character that may be seen occasionally in 

 other voles, e.g., Phaiomys. 



The anterior palatal foramina are usually moderately large, 

 but in forms showing marked fossorial specialization of the skull 

 they are much reduced in size. From the posterior end of each 

 foramen a well-marked groove, the lateral palatal groove, runs 

 back on each side of a median elevated tract and crosses the 

 maxillo-palatine suture in the neighbourhood of m^ ; the groove 

 may terminate here in one of the posterior palatal foramina ; 

 but, if the 250stero-lateral bridges are well developed, it usually 

 continues past the foramen, deepens behind it, and passes under 

 (dorsal to) the postero-lateral bridge to effect, as a rule, a junction 

 with the postero-lateral pit (" post-palatine fossa "). 



The posterior portion of the palate presents characters of 



