448 HENRY C. TRACY 



may mention his suggestion that forms like Megalops may be 

 transitional stages between the primitive type and the clupeoid 

 type. As de Beaufort remarks, however, it is evident that the 

 connection between the ear and swimbladder has developed in- 

 dependently in different groups of fishes. Apparent resemblances 

 in these structures, as found in existing genera, indicate little 

 regarding relationship or even the phylogenetic development . of 

 the ear-swimbladder relation in any given group of fishes. 



De Beaufort describes stages in the embryonic development of 

 these structures in the herring. He shows that the bony cap- 

 sules are formed not by a 'hollowing out' process of the bones 

 in which they are enclosed in the adult, but by a new process of 

 bone development from connective tissue around the mem- 

 branous vesicle of the precoelomic diverticulum. 



TERMINOLOGY 



The following terms are used throughout this paper in re- 

 ferring to the anatomical relations of the cranial structures, 

 exoccipital (for occipitale laterale), epiotic, prootic (for petrosal) : 

 pterotic (for squamosal), opisthotic (for intercalar), and sphe- 

 notic (for postfrontal or postorbital ossification). Median 

 designates the midvertical plane or structures lying within it; 

 medial indicates the opposite of lateral, i.e., toward the middle. 

 In referring to the channels in the tissue around the mem- 

 branous labyrinth and the brain, the term ' perilabyrinthine' is 

 used instead of perimeningeal or perilymphatic. 



MATERIAL 



This investigation is based on the study of the more common 

 American clupeoid fishes. Adult specimens of the following 

 species were examined: Alosa sapidissima (shad), Pomolobus 

 pseudoharengus (alewife), Pomolobus aestivalis (summer her- 

 ring), Pomolobus mediocris (hickory shad, fall herring), Bre- 

 voortia tyrannus (menhaden) . The shad were bought in local 

 markets; the specimens of the other species were obtained from 

 the Marine Biological Laboratory, where they had been pre- 



