THE CLUPEOID CRANIUM AND THE SWIMBLADDER 455 



part of the basioccipital. It thus forms the roof of the hinder 

 part of the saccular cavity. The plates of the two sides exclude 

 the basioccipital completely from the foramen magnum and the 

 cerebral cavity, and form a trough in which the brain stem 

 rests. Near the anterior edge of the triangular plates are two 

 oblique foramina (fig. 4) ; one, anterior to the other and much 

 the smaller, opens to the upper lateral part of the saccular cavity 

 and gives passage to the ninth nerve; the other is the entrance 

 to a canal which is directed obliquely backward and outward 

 and forms the exit of the tenth nerve. Posteriorly are the exits 

 of the two occipital nerves. 



The ventral part of the exoccipital bone forms a considerable 

 portion of the floor and sides of the saccular cavity. Its most 

 conspicuous feature is a small fusiform bulla through which the 

 diverticulum of the swimbladder passes. Dorsolateral to that 

 structure is the large oval opening of the canal of the tenth 

 nerve; medial to it and almost at the edge of the auditory fora- 

 men is a small opening for the ninth merve (fig. 5). 



3. The prootic bone 



The conspicuous feature of the prootic bone is the large spher- 

 ical bulla or osseous capsule which lodges the anterior vesicle of 

 the swimbladder diverticulum. Looking first at the cerebral 

 surface of the bone (figs. 6 and 7), one observes that it is thick 

 and massive medial to the bony capsule (medial plate) and 

 forms with the corresponding part of its fellow a rounded ridge 

 of bone which extends transversely across the floor of the an- 

 terior part of the cerebral cavity like a raised threshold. The 

 sixth nerve passes vertically through this part of the bone to the 

 eye-muscle canal below (figs. 6 and 8). Posteriorly, on the 

 cerebral surface of the bone, just behind the capsule, is a deeply 

 concave depression which, in the articulated skull, is continuous 

 with the dorsolateral concavity of the basioccipital and com- 

 pletes the saccular cavity anteriorly. 



Lateral to the osseous capsule, two bony plates are given off 

 (fig. 6). The lower plate, the lateral wing of the prootic, is con- 



