THE CLUPEOID CRANIUM AND THE SWIMBLADDER 467 



of bone which separates the orbital from the ventral surface of 

 the prootic is greatly developed (fig. 2). As a result of these 

 differences in the relative bone development in this region, the 

 third and fifth nerves of B. tyrannis and of A. sapidissima appear 

 to exit from the skull through canals instead of mere foramina as 

 is the case in P. pseudoharengus. 



The morphology of the relation of the swimbladder diver- 

 ticulum to the cranium is a difficult problem. The suggestion 

 of de Beaufort has been discussed above. Tysowski has de- 

 veloped an elaborate theory, the factual basis of which seems to 

 be the relation of the septum in the anterior bony capsule to 

 the bone to which it is attached. This septum, being in inti- 

 mate connection with the bone of the capsule— its structure, in 

 fact, continuous with it — and being covered by a reflection of 

 periosteum from the bone surface, is (according to Tysowski's 

 view) morphologically a part of the bone itself. 



These facts persuaded Tysowski that the swimbladder diver- 

 ticulum does not pierce the floor of the skull and lie next to the 

 labyrinth in a cavity hollowed out in the prootic bone, but that 

 the septum is a part of the cranial floor, and the swimbladder 

 vesicle in the chamber below it is as much outside the skull as in 

 other forms. Similarly the wall of the bony capsule is not a 

 simple structure, but is of double origin, the part above the 

 septum developing differently than the part below. He con- 

 siders that the floor of the skull in the basisphenoid region gives 

 off a process dorsally, which bends over medially and so forms 

 the upper chamber which he says is comparable with the saccular 

 ■cavity. The inferior chamber is formed similarly by a lateral 

 process (presumably from the basisphenoid bone) which bends 

 ventrally to enclose the anterior membranous vesicle of the 

 swimbladder. 



Tysowski's theory outlined above presents an interesting 

 hypothesis in that the ear-swimbladder mechanism in the Clu- 

 peoids can easily be correlated with the simpler, more primitive 

 type. With this theory as a basis, we might assume a nearly 

 complete series of transition stages from the primitive relation of 

 apposition (as in the case of the ear and swimbladder of Sparidae 



