466 HENRY C. TRACY 



lateral-line enlargement which occupies the temporal foramen; 

 another appears to go to the commissural lateral-line canal; 

 probably others contribute to the plexus formed by the ramus 

 recurrens facialis. 



Two occipital nerves leave the skull through the upright 

 portion of the exoccipital bone. 



DISCUSSION 



The relations of the bony elements of the skull to each other 

 and to the precoelomic diverticulum of the swimbladder and to 

 the membranous labyrinth which have been described in this 

 paper are essentially the same for all the other American clupeoid 

 species which I have examined. They also seem to correspond 

 to those of the European species in which the details of the ear- 

 swimbladder relation have been investigated. Certain of the 

 more conspicuous features in which other American species differ 

 from P. pseudoharengus may now be briefly summarized. 



In general, differences in skulls of the other species seem 

 dependent chiefly upon differences in the degree of ossification. 

 The lateral cartilage plate is more or less completely ossified in 

 the adult skulls of the other American species; in Brevoortia and 

 Alosa, about the only remains of this structure is that part 

 which contains the anterior semicircular canal. In these forms 

 also the hook-like process of bone from the epiotic around the 

 upper end of the anterior semicircular canal forms a complete 

 ring. 



On the external surface of the skull of Pomolobus mediocris, 

 the posterior osseous capsule is only slightly visible; in Brevoortia 

 tyrannis the surface of this capsule is completely covered with 

 bone except for an area in the epiotic fossa so small as to be 

 visible only with a microscope; in Alosa sapidissima the surface 

 of this capsule is entirely obliterated by the development of bone. 

 In most of these species, also, the bone development in the 

 anterior cranial floor is much more extensive than in P. pseudo- 

 harengus. The lateral parts of the basisphenoid bone are 

 thickened into massive processes and the sharply projecting edge 



