INTRODUCTORY 



237 



of the skin ; these bones are known as dermal bones or mem- 

 brane bones, and many of them persist in the skull and pectoral 

 girdle of the higher terrestrial vertebrates. Thus we have the 





Fig. 19. — Dermal denticles of the Dog-fish (Scy Ilium canicula). A, from 

 above ; B, from below ; C, in section ; b, bony base ; d, dentine ; en, enamel ; 

 ep, epidermis. 



remarkable fact that the frontal and parietal bones of the 

 human skull are originally derived from the bony scales on the 

 head of fishes, and at a still earlier stage of evolution from the 

 dermal denticles on the skin of the head of the shark-like 



