THE VERT EBRAT A 225 



complete series, and it is difficult to know whether slight 

 differences among isolated teeth are due to their belong- 

 ing to different species of sharks or to different parts 

 of the jaw of the same species. 



Like other vertebrate teeth, those of the sharks consist 

 of a root, buried in the soft tissues, and a crown which 

 is exposed. Up to the Jurassic period, shark's teeth 

 have shallow, undivided roots (Fig. 63, b, c)\ from the 

 Cretaceous period onwards, forms with deep, divided 

 roots occur (Fig. 63, d, e, /). 



The tooth is mainly composed of dentine, a calcareous 

 tissue full of fine tubules into which pass processes 

 from the soft tissues of the central pulp-cavity (Fig. 64). 

 The crown has its surface covered with a thin layer 

 of enamel, a much denser material than the dentine. In 

 the fossil state, teeth are usually highly phosphatized, 

 and may acquire a black colour. 



Sharks' teeth are among the most indestructible of 

 fossils, and worn examples are common among derived 

 fossils, 



In one remarkable Palaeozoic group (Cochliodontidas 

 and Edestidae) the teeth of the middle line of the jaws 

 do not fall out but curl in under the front of the jaw 

 as the new teeth push them forward. In the latest 



PIG. 63. FISH TEETH AND SCALES (continued). 



Agassiz, Rheetic, Tooth. (xf.) h, Acrolepis semigranulosus 

 Traquair, Lower Carboniferous. A; single ganoid scale, X j, surface 

 pattern omitted, i, Lepidotus minor Agassiz, Purbeck beds (Upper 

 Jurassic). Part of scaly armour of trunk. (X^.) j, Ptychodus 

 mammillaris Agassiz, Upper Cretaceous. Tooth, surface view ; 

 f, profile. (Xf.) The area shown dotted is part of the crown and 

 vS*>onld be shown with fine, more or less concentric lines, a, A, After 

 Traquair ; b, f, /, g, after Agassiz ; d, e, i, /, after Smith Woodward, 



