27 



Figs. 20-31. Mode of evolu- 

 tion of (teeth and) dermal defences. 



20. Shagreen denticle of shark, X 30, 

 cross section. (After HOFER.) D. 

 Dentine. D'. Derma. E. Enamel. 

 '. Epidermis. EO. Enamel organ. 

 PC. Pulp cavity, showing nutritive 

 tubules passing into the dentine. 



21. Shagreen denticle (" placoid 

 scale ") of Greenland shark, Lcemar- 

 gus, viewed from the side and (A} 

 top, enlarged. 22. Shagreen denti- 

 cles of shark, Scyllium, showing 

 mode of arrangement, x 30. 23. 

 Shagreen of sting-ray, Urogymnus, 

 nat. size. (After SMITH WOOD- 

 WARD.) 24. Ganoid dermal plates of Lepidosteus. A. Inner face of ganoid plates, 

 showing tile-like device of interlocking. 25. Variation of ganoid plates in Aetheolepis. 

 (After SMITH WOODWARD.) Plates from different regions vary in outline from cir- 

 cular to lozenge shape. 26. Coalesced ganoid plates of the siluroid Calhchlhys. 

 27. Jaw of Port Jackson shark, Cestracion. 28. Dental plate of extinct cestraciont (?), 

 Sandalodus. 29. Dental plates of jaw of sting-ray, Trygon (?). 30. Dental plates 

 of eagle-ray, Myliobatis. 31. Scales of Teleost. A. A single scale enlarged. 



24 



