4 
FIG. 20 
Figs. 20-31. — Mode of evolu- 
tion of (teeth and) dermal defences. — 
20) Shagreen denticle of shark, x 30,+ 
“cross section, (After HOFER.) JD. 
Dentine. DD’. Derma, £#. Enamel. 
&'. Epidermis. £O.Enamelorgan, = 
PC. Pulp cavity, showing nutritive 
tubules passing into the dentine, — 
(2p. Shagreen denticle (“placoid) — 
scale") of Greenland shark, 
gus, viewed from the side and (A), 
top, enlarged. 22, Shagreen denti-. 
cles of shark, Scy/dium, showing. 
mode of arrangement. X 30, 23. 
. Shagreen of sting-ray, Urogymnus, 
nat.-size. (After SMITH WoopD- 
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 pares 
(27. Jaw of Port Jackson shark, Cestracion, 28. Dental plate of extinct cestraciont (?), 
“Sandalodus. 29. Dental plates of jaw of sting-ray, hte (?). 30, Dental plates 
of eagle-ray, Myliobatis, 31. Scales of Teleost. A. A single scale enlarged. 
24 
