THE CLUPEOID CRANIUM AND THE SWIMBLADDER 471 



ture of the macula acustica utriculi and its functional significance 

 in connection with these mechanical relations will be discussed 

 more fully in the paper to follow. 



SUMMARY 



1. The precoelomic diverticulum of the swimbladder divides 

 into two branches which are conducted in a cartilage tube to the 

 exoccipital bone on each side of the skull. 



2. In canals in the bones at the base of the skull the diver- 

 ticulum bifurcates; one branch passes laterally to the pterotic 

 bone where it expands to form the posterior membranous vesicle 

 contained in a spherical bony capsule ; the other branch passes to 

 the prootic bone where it forms the anterior membranous vesicle 

 also surrounded by a bony capsule. 



3. The osteological structure of the clupeoid cranium conforms 

 to the type of the lower teleosts. Specialized features of the 

 clupeoid cranium are : 



a. The deep saccular recess partially covered by a triangular 

 plate of the exoccipital bone. 



b. The auditory foramen between the basioccipital, pterotic, 

 and prootic bones. 



c. The temporal foramen between the frontal, parietal, epiotic, 

 and pterotic bones, occupied by a bay-like expansion of a lateral- 

 line canal. 



d. The lateral recess between the sphenotic, pterotic, prootic 

 bones and the lateral wing of the frontal bone. 



e. The falciform process of the prootic bones. 



/. The great ventral and posterior extension of the wings of 

 the parasphenoid bone. 



g. The spherical osseus capsule in the prootic bone which 

 communicates with the cavum cranii through a slit-like transverse 

 fenestra. 



h. The spherical osseus capsule in the pterotic bone. 



i. The fusiform bulla in the exoccipital bone. 



j. The inner wall of the lateral-line canals belonging to the 

 lateral wing of the frontal bone is absent; the canals therefore 

 lie in the loose tissue of the lateral recess. 



