.^TS HUGH 



OLOGY DEPARTMENT 



NGTON SQUARE cc'^^j^ FACE FROM FISH TO MAN 



off or shed. Nevertheless the teeth of the higher 

 vertebrates probably arose not from horny epi- 

 thelial teeth like those of the modern lampreys, 

 but from enamel-covered shagreen denticles such 

 as covered the whole body of Lanarkia, one of the 

 Scotch Silurian ostracoderms. In the sharks (Fig. 

 60B, C, D) each little shagreen denticle on the 

 surface of the skin consists of a little cone in 

 which a porcelain-like layer of "enamel" is laid 

 down between the epithelial covering and the 

 pulp cavity. These shagreen denticles, together 

 with the stratified bony deposits in the deepest 

 layers of skin, gave rise not only to the teeth of 

 higher vertebrates but also to the enamel-covered 

 bony plates that cover the braincase, the bony 

 tooth-bearing plates that cover the primary 

 cartilaginous jaws and the bony tooth-bearing 

 plates on the roof of the mouth, both in the air- 

 breathing, lobe-finned fishes and in their successors, 

 the earliest amphibians. 



These enamel-covered plates were also homolo- 

 gous with the bony ganoid scales on the surface of 

 the body. 



Thus we are again reminded of the remarkable 



potentialities of the many-layered skin in the 



100 



