HEADS AND TAILS. 49 



symmetrical plates. These removed, as we have 

 already remarked, would reveal a skull differ- 

 ing only in some smaller particulars from that of 

 the dog-fish. In the cavity of the mouth we 

 should see, furthermore, thin plates of bone 

 investing the arches of the gills, and ensheathing 

 the floor of the cartilaginous brain-case. How 

 did this external armour plating come to hel 

 We cannot say for certain. Possibly, even 

 probably, by the fusion or welding together of 

 numerous "placoid scales" or shagreen denticles. 

 The advent of these bones marks a very im- 

 portant epoch in the history of the development 

 of the skull. It is well, therefore, to make 

 careful note of their presence at this stage, and 

 of the relations which they bear ; for from this 

 time onward the part which they play in the 

 protection of that all-important organ the brain, 

 and the perfection of what we may call the 

 machinery of mastication, becomes greater and 

 greater, ending only with ourselves. 



Our next stage in the development of the 

 skull, then, we find in the lung-fishes, where 

 bony centres have established themselves in the 

 ear-capsules — till now cartilaginous, and there is 

 a slight advance in the bone-sheath of the mouth 

 parts. The skull of that remnant of an ancient 

 house, the bichir or polypterus, adds more links 

 to the chain. The quantity of bony matter has 

 now greatly increased, but the cranium is still 

 cartilaginous. As we work higher and higher, 

 however, this cartilage becomes less and less 

 conspicuous till, if we traced the development 

 of the skull into the higher vertebrated animals, 

 D 



