3io 



VERTEBRATA. 



proceeding from a common base, and all divided transversely 

 into numerous short pieces. The soft rays occur in many fishes 

 in different fins, but they are invariably found in the caudal 

 fin or tail (fig. 275, c). The rays of the median fins, whatever 

 their character may be, always articulate by a hinge-joint with 

 the heads of the interspinous bones. 



The skull of the bony fishes is an extremely complicated 

 structure, and it is impossible to enter into its composition 

 here. The only portions of the skull which require special 

 mention are the bones which form the gill-cover or operculum. 

 For reasons connected with the respiratory process in fishes, 

 there generally exists between the head and the scapular arch 

 a great cavity or gap on each side, within which are contained 

 the branchiae. The cavity thus formed opens externally on 

 each side of the neck by a single vertical fissure or " gill-slit," 

 closed by a broad flap, called the "gill-cover" or " operculum," 

 and by a membrane termed the " branchiostegal membrane." 



The gill-cover (fig. 276, /, o, s, /') is composed of a chain of 

 broad flat bones, termed the opercular bones. Of these, the 



Fig. 276. Skull of Cod (Morrhua vulgaris) Cuvicr. a Urohyal; b Basihyal; f 

 Ceratohyal ; d Branchiostegal rays ; / Prae-opcrculum ; a Operculum proper ; s Sub-oper- 

 culum; /' Inter-operculum ; m Mandible; " Intermaxillary bone. 



innermost articulates with the skull (tympano-mandibular arch), 

 and is called the " pne-operculum ; " the next is a large bone 



