452 HENRY C. TRACY 



tic osseous capsule, or bulla. The main branch of the diver- 

 ticulum, from the angle between the bones above mentioned, 

 continues forward in a canal in the prootic. In this bone the 

 diverticulum ends by forming a membranous vesicle (anterior 

 or prootic). The bone around the vesicle forms the anterior, 

 or prootic bulla, or osseous capsule. 



The cartilage canals in which the extracranial portions of the 

 diverticulum are enclosed form a Y-shaped structure (fig. a). 

 The stem of the Y is a median, unpaired structure; in Alosa it 

 is a short, thick cylinder with its posterior surface forming a 

 vertical transverse plane and facing the anterior end of the 

 swimbladder; this surface is smooth and circular, and around 

 its edge is beset with several large blunted spines. In Pomolo- 

 bus, the posterior surface is concave. To these spines and 

 around the margin of its smooth face is attached the aponeurotic 

 sheath which invests the anterior end of the swimbladder. An- 

 teriorly, this cylinder-like structure is continued off into two 

 cartilage tubes which diverge from each other to form the arms 

 of the Y which embrace the aorta between them. These tubes 

 gradually diverge from each other, pass obliquely over the sur- 

 face of the parasphenoid wings just in front of their tips, then 

 over the aponeurosis which clothes the occipital musculature 

 and extend cephalad to the side of the exoccipital bone. They 

 are slightly flattened as they are applied to the surface of the 

 aponeurosis; one of these in a shad measured 0.38 mm. in one 

 dimension and 0.5 mm. in the other. 



On the surface of the exoccipital bone is a groove (fig. 5) 

 which begins toward the lower posterior angle of the bone and 

 slants upward and forward. The groove gradually deepens as it 

 passes upward, and finally ends in the opening of the canal 

 through which the diverticulum passes. The ends of the car- 

 tilage tubes are very oblique and are applied to the sides of the 

 groove and the edge of the mouth of the canal into which it 

 leads. Thus the groove in the bone is made into a complete 

 canal by the application to it of the oblique end of the cartilage 

 tube, and through this canal the diverticulum of the swimbladder 

 enters the exoccipital bone. 



