VERTEBRATA. 131 



(ossification) or by substitution. The bony cranium (unlike 

 the cartilaginous cranium) is not a continuous wall, but is 

 composed of separate bones firmly united together, the 

 number varying between wide limits, being most numerous 

 in the lower and reduced by fusion in the higher forms. 



In the sharks the facial skeleton is very simple, being 

 represented by the upper and lower jaws, and by a few car- 

 tilages supporting the lips. The upper jaw is not firmly 

 united to the cranium, but is held in position by muscles 

 and ligaments, while the lower jaw is hinged to the upper, 

 and not to the cranium. Comparisons, which cannot be 

 described here, show that the upper jaw of the shark is not 

 the same as the upper jaw in the other vertebrates. In 

 them numbers of other bones are added to the skull, and 



FIG. 58 Skull of cod. (After Hertwig.) The dotted portion is the 

 equivalent of the upper jaw of the shark (Fig. 57). 



the upper jaw of the shark is only comparable to three pairs 

 of bones, known to the anatomist as the palatines, ptery- 

 goids, and quadrates. 



The visceral skeleton consists of bars of cartilage on either 

 side of the throat between the gill-slits, the series being 

 united below (Fig. 57). These gill-bars serve to keep this 

 region, weakened by the openings, from collapse. The most 



