THE CLUPEOID CRANIUM AND THE SWIMBLADDER 



451 



the details of this structure in different species. In Alosa (fig. a) 

 and Stolephorus it passes forward a few millimeters from the 

 anterior end of the swimbladder and enters a cartilage canal in 

 which it almost immediately bifurcates. In Pomolobus, the 

 branches come off directly from a slight bulbous enlargement of 

 the tapering end of the swimbladder. Each of the branches, 



Fig. b. Drawing to show relations of tlie membranous vesicles of the pre- 

 coelomic diverticulum to the bones at the base of the skull. Based on figure 8. 

 The ventral segments of the bony capsules are represented as having been cut 

 out. Diagrammatic. Abbreviations on page 473. 



enclosed in a continuation of the cartilage canal, runs forward, 

 one on each side, under the occipital portion of the skull and 

 enters a canal in the exoccipital bone (fig. 2). The canal bulges 

 somewhat here (P. pseudoharengus) to form a fusiform enlarge- 

 ment ; it then passes forward to the angle where the exoccipital 

 prootic and pterotic bones meet (fig. b). At this point, it 

 gives off a lateral branch which enters a nearly spherical cavity 

 in the pterotic bone which it expands to occupy, thus forming 

 the pterotic or posterior membranous vesicle. The part of the 

 pterotic bone which encloses this vesicle is the posterior or ptero- 



