Skull of Fishes. 17 



laginous box, as in lampreys and sharks; in others 

 this cartilaginous box is covered and protected by a 

 series of bony dermal plates, as in the sturgeon, or 

 the whole skull may be made up of a number of 

 closely articulated and perfectly united bony pieces 

 as in the cod (fig. 8). In this case it must be re- 

 membered that these bones are of a twofold nature 

 first, the ossified pieces of the cartilage of which the 

 primitive skull consists, and which surrounds the dif- 

 ferent apertures and nerves, forming principally the 

 lateral walls of the skull ; secondly, the ossified plates 

 of membrane which are the equivalents of the dermal 

 bony plates of the sturgeon, and which chiefly make 

 up the roof and floor bones. 



Besides the skull or brain-case proper, the % head of 

 a fish consists of four other series of bones, as can be 

 seen in fig. 8. These are, first, those of the upper jaw 

 arch, sometimes seven in number on each side ; 

 secondly, those of the lower jaw arch, sometimes four 

 or five pairs ; thirdly, those of the gill arches, four, five, 

 or six pairs of arches on each side, each consisting of 

 about four pairs of bones, and bearing the gills These 

 three series are chiefly ossifications in the system of 

 visceral arches before referred to. Besides these, there 

 is a fourth group of bones, those of the operculum or 

 gill cover, which overlap and cover the gill arches ; 

 of these there are four or more, making up the gill 

 cover on each side. It is thus not to be wondered at 

 that the skeleton of the head of a fish presents an 

 appearance of great complexity. 



The limbs of fishes are converted into fins, and of 

 these there are usually two pairs. The fore limbs, or 

 c 



