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 
papilla 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- 
