316 THE VERTEBRATE SKELETON. 



middle line by a pair of holes, the anterior openings of the 

 Eustachian canals \ while just in front of these and a little 

 further removed from the middle line are the anterior openings 

 of the carotid canals. Bristles passed in through the posterior 

 openings of the carotid canals will emerge here (fig. 60, A, 27). 

 In front of the basitemporal the base of the cranium is formed 

 by the rostrum (fig. 60, A, 31), or thickened basal portion of 

 the iiiterorbital septum ; this bears two prominent surfaces 

 with which the pterygoids articulate. In some kinds of 

 duck these surfaces are borne by well-marked basi-pterygoid 

 processes. 



(d) The side of the cranium. At the base of the posterior 

 end is seen the deep tympanic cavity. The dorsal part of 

 this is divided by a vertical partition into two halves ; of these 

 the more anterior is the larger, and forms a deep funnel- 

 shaped cavity, the posterior opening of the Eustachian 

 canal (fig. 60, B, 26). A bristle passed into this opening- 

 emerges through the anterior opening of the Eustachian canal. 

 The more posterior of the two is the fenestral recess (fig. 60, 

 B, 28), and is in its turn divided by a slender horizontal bar 

 into a dorsal hole, the fenestra ovalis, and a ventral hole, 

 the fenestra rotunda. During life the fenestra ovalis lodges 

 the proximal end of the columellar chain. Lying at the 

 outer side and slightly dorsal to the tympanic cavity is a deep 

 depression, the lateral tympanic recess, and immediately 

 in front of this is the articular surface for the quadrate. 

 The tympanic cavity is bounded below by the basitemporal, 

 posteriorly by the exoccipital, and above by the squamosal, a 

 membrane bone, which roofs over a good deal of the side of 

 the cranium, and bears ventrally a prominent surface with 

 which the quadrate articulates. Just in front of this is a 

 large round hole, the trigeminal foramen (fig. 60, B, Y), 

 behind which the squamosal is drawn out into a short process. 



In front of the squamosal there is a prominent forwardly- 

 projecting postfrontal process (fig. 60, 8), which ossifies 



