ARVICOLA 399 



oblique with age ; its upper part being pressed forwards, at the 

 expense of the interparietal, so that the condyles appear wholly 

 or in part in the dorsal view. Diastema long, in profile rather 

 high-pitched behind the incisors. Anterior palatal foramina 

 very small, their length scarcely half that of the diastema, com- 

 mencing far behind incisors and terminating considerably in front 

 of the molars. Molars very robust and hypsodont, filling great 

 capsules which rise high in the floors of the orbit and braincase, 

 that of ni^ obstructing the mouth of the sphenorbital fissure; 

 presphenoid reduced to a slender rod of bone. 



Palate essentially as in Microtus ; its posterior median sloping 

 septum rather short, broad, and low. Mesopterygoid space rather 

 small. Pterygoid fossaj deep and extensive, their floors con- 

 siderably dorsal to the ventral surface of the basisphenoid. 

 Auditory bulla; rather small, but little inflated, the basioccipital 



Fig. 109. — Cheek-teeth of Arvirola amphibius Linnwus 

 CrowTi views : a. right upper, h. right lower molars. 



remaining relatively wide between the bullae ; cavity of bulla 

 simple, its walls strengthened by a light network of bony threads 

 •which thickens posteriorly into a definite bony sponge in the 

 hinder part of the tympanic wall of the bulla ; mastoid portion 

 and tegmen tympani not inflated, the tegmen articulating by a 

 squamous facet with the alisphenoid in front ; stapedial artery 

 enclosed in a bony tube, which passes through the stapes. 



Mandible essentially as in Microtus, but the horizontal ramus 

 greatly thickened for the accommodation of the robust teeth ; 

 the incisor produces a well-marked hump on the outer surface of the 

 condylar process a little above the level of the dental foramen; above 

 the incisor hump the condylar process is fairly sharply inflected ; 

 angular process usually well developed but rather short and stout, 

 its inferior border thickened to form a broad surface for the 

 attachment of the anterior or superficial portion of the masseter 

 lateralis muscle. In adults the growing bases of w^ and wij 

 produce rather well-marked swellings on the infero-lateral surface 

 of the ramus, just below the crista masseterica, and the capsule of 



