SKULL 17 



first and second branches of the fifth nerve ; the outer division 

 between the tooth capsules and the ahsphenoid serves for the 

 passage of the internal maxillary artery, which often grooves the 

 outer side of the alveolar capsule of ?u^. 



The alisphenoids are very large ; of each the ascending process 

 forms a large part of the anterior wall of the braincase, rising 

 in the orbit nearly to the level of the post- orbital process of the 

 squamosal and there articulating with the frontal ; the descending 

 process forms an ectopterygoid plate, greatly developed in all 

 Microtinae, which articulates with the squamosal and auditory 

 bulla behind, and bridges the foramen ovale to serve as a buttress 

 supporting the hinder end of the alveolar portion of the maxilla. 

 The horizontal plate is anchylosed with the basisphenoid as usual ; 

 it is pierced at its base by a foramen for some of the cerebral 

 vessels in front of the very shallow pituitary fossa, and externally 

 by the large foramen ovale. Just below the front edge of the 

 foramen ovale and external to the ectopterygoid plate is a foramen 

 by which the internal maxillary artery enters the outer division 

 of the sphenorbital fissure. There is no distinct foramen rotun- 

 dum, the second branch of the fifth nerve emerging with the first 

 through the sphenorbital fissure. The orbito-sphenoids are 

 small, are completely anchylosed with the presphenoid, and are 

 pierced by the small optic foramina, as usual. Posteriorly the 

 basisphenoid is often notched for the passage of the internal 

 carotid artery ; in Prometheomijs the notch is represented on each 

 side by an oblique tunnel, the mouth of which is a conspicuous 

 feature in the floor of the braincase; small passages from the 

 anterior wall of this tunnel place it in communication with the 

 vascular passages through the base of the alispheuoid and also 

 with the transverse canal, which is constantly present in the 

 Microtinte and gives passage to a vein that passes from one 

 pterygoid fossa to another through the body of the basisphenoid. 



The auditory bullae are always well developed. When least 

 modified they have thin papery walls; when more specialized 

 their walls are strengthened by bony threads developed in folds 

 of the mucous membrane lining the cavity of the middle ear; 

 and in their most modified condition the walls are formed by 

 a compact mass of spongy bone, and the teg men tympani and 

 mastoid portions are considerably inflated. The external meatus 

 becomes tubular in aquatic forms, and is often much straitened 

 in fossorial genera. The stapedial artery is often enclosed in a 

 bony tube which passes through the stapes. 



The mandible always possesses distinct coronoid and angular 

 processes, although the latter are reduced in some genera. The 

 horizontal ramus is characteristically stout, and is considerably 

 thickened for the accommodation of the hypsodont cheek-teeth. 

 At the symphysis the mandibular rami are united, as in other 

 Muridse, merely by ligament. 



