vii.] THE EXTERNAL SKELETON, 273 



singularly dense and hard, well suited for breaking up their 

 stony prey. The jaws of the fish are shaped like a parrot's 

 beak (whence its name) ; but it is a beak which singularly 

 and interestingly differs in structure from that of a bird. 



FIG. 250. VERTICAL SECTION OF THE LOWER JAW OF A SHARK (Lamna), 

 showing the uppermost erect tooth with the others below in various stages, 

 each ready to come successively into place when its predecessor is removed. 

 (After Owen.) 



A bird's beak is, as we have seen, a modification of the 

 epidermis. This fish-beak, however, is a truly enderonic 

 structure, and calcareous, not horny. It is, in fact, formed 

 of an immense number of small elongated teeth, which, 

 closely packed side by side, are attached by their proximal 

 ends to the surface of both the upper and lower beak-shaped 

 jaws. As these jaws with the annexed denticles are worn 

 away by use at their margins, both are replaced by a com- 

 mon down-growth of bone and teeth in the upper jaw, and by 

 a common upgrowth of bone and teeth in the lower one. 



FIG. 251. Sit>E VIEW OF THE PRE-MAXILLA OF A PARROT-FISH (Sc<irus\ 

 showing the closely apposed denticles which encrust it. (After Owen.) 



33. The STRUCTURE of man's teeth supplies us with a 

 key by which we may understand that of Vertebrates gene- 

 rally ; nor is there any real increase of complexity as to the 

 number of the constituent materials, though the dentine may 



T 



