EVOLUTION OF SCALES 



25 



members of the shark group the denticles are scattered 

 over the body without traces of metameral arrangement 

 (Fig. 23) ; in others they acquire a segmental position 

 (Fig. 22). Usually the denticles possess very definite 

 shapes and regional characters ; their basal portion, where 

 implanted in the skin, may thus become of enlarged size 

 and regular outline (Fig. 21 A), their projecting cusps 

 tapering, blunted (Fig. 23), or branched. Sometimes the 

 fusion of contiguous denticles may occur (as in the en- 

 larged blunted denticles of Fig. 23). 



The evolution of the more perfect body armouring of 

 fishes from shagreen denticles has not been followed in 

 minor details. It appears, however, that the. calcifica- 

 tion of the skin which occurs superficially in the dermal 

 papillae of the shark may in other fishes be traced oc- 

 curring in deeper and deeper layers of the derma : the 

 papillae at the surface accordingly lose their functional 

 importance, and tend to disappear, while the calcified 

 tissue of the derma — representing morphologically the 

 basal region of the denticles — is coming to occupy more 

 and more definite tracts. These processes have already 

 taken their origin within the group of sharks. 



An interesting condition in the subsequent evolution of 

 the dermal armouring is illustrated in Fig. 25, and has 

 been described by Smith Woodward. The circular bone 

 plate of the figure is a calcified dermal tract which still 

 retains, scattered generally over its surface, traces of 

 shagreen tubercles : from this shark-like condition a 

 well-marked gradation in the form of the derm plates 

 may be traced in different body regions of the same 

 fish : according to metameral needs there are acquired 

 rectangular or lozenge-shaped outlines. In Fig. 24 these 

 bone or " ganoid " plates are seen to constitute a com- 



