OUR ANCIENT RELATIVES 



the complex jaws, consisting of the primary or 

 originally cartilaginous upper and lower jaws plus 

 their bony dermal covering, and second, the sliding 

 bony covers of the gill chamber. 



In the modern sharks the skeleton is stiffened 

 by calcium carbonate rather than by phosphate 

 of lime, the skin is stiffened chiefly by the shagreen 

 or little teeth on its surface and the skeleton as a 

 whole remains in a low stage of evolution. 



On the other hand, in the ancient lobe-finned 

 ganoid fishes, which stand much nearer to the 

 direct line of human ascent than do the sharks, 

 phosphate of lime is deposited by true bone-cells 

 and the skull comprises a bony mask and a bony 

 braincase as described above. 



The whole surface of the mask (Fig. 11) is cov- 

 ered by a thin enamel-like layer, smooth and 

 shining, called ganoine. 



The jaws of the ancient ganoids, well covered 

 both on the inner and outer sides by an armor of 

 bony dermal plates, carried large sharp teeth with 

 deeply infolded or labyrinthine bases (Fig. 18A). 



There is every reason to regard these mail-clad 

 robbers as lying not far off the main line of ascent. 

 The alligator-gar of the lower Mississippi system, 



