THE CLUPEOID CRANIUIVI AND THE SWIMBLADDER 459 



(figs. 3, 4 and 6). In consists of a narrow transverse slit, slightly 

 crescent-shaped in some species. Its edges, at its lateral end 

 and sides, protrude upward slightly to form a raised lip around 

 its margin. This lip gradually becomes lower toward the 

 medial end of the fenestra where it is flush with the surface of 

 the surrounding bone. The utriculus rests on the edges of this 

 lip to which it is attached by delicate connective-tissue fibers. 



The cavity of the capsule is subdivided into an upper and a 

 lower chamber by a nearly horizontal septum of elastic tissue, 

 the elements of which are in the form of flattened plates (figs. 5 

 and 9, SC). Each surface of this septum is covered by a re- 

 flection of the periosteum from the inner surface of the bone of 

 the corresponding chamber. The septum is closely attached to 

 the bone by means of radial connective-tissue fibers which pass 

 from the elastic plates of the septum into minute canals in the 

 bone substance. The lower chamber contains the anterior mem- 

 branous vesicle of the swimbladder. The upper chamber was 

 erroneously supposed by Weber and Ridewood to be occupied by 

 a diverticulum from the utriculus; it is merely a tissue space 

 which communicates with the subcerebral perilabyrinthine canal 

 through the fenestra. 



This bone forms the lateral part of the posterior portion of the 

 cranial wall. It is a relatively large, massive bone in which is 

 imbedded the posterior osseous capsule (figs. 3 and 11) con- 

 taining the posterior vesicle of the swinibladder diverticulum. 

 The lateral surface of the pterotic is partly covered anteriorly 

 by a thin scale of bone which is an extension of the lateral wing 

 of the frontal bone (fig. 1, LWF); posteriorly, the lateral surface 

 is deeply indented by the lower portion of the epiotic fossa 

 {EPF). The posterior osseous capsule protrudes into the epiotic 

 fossa and also exposes its surface slightly on the face of the bone 

 below the fossa. The lateral semicircular canal encircles the 

 bony capsule near its equator. 



