CHAPTER XVIII 



THE TEETH 



Teeth and the denticles (or placoid scales) of the dogfish are 

 identical in that they consist essentially of a hollow cone of 

 dentine, inside which is a pulp-cavity, and outside which is a 

 layer of enamel. The dentine is formed from the mesoderm 

 of the skin, and the enamel is produced from the overlying 

 ectoderm. The denticle or tooth is formed below the surface 

 of the skin, and is subsequently erupted through it. In the 

 dogfish the denticles are not restricted to the borders of the 

 mouth, but occur all over the surface of the body. In a few 

 bony fish such as Polypterus, Lepidosteus, and catfish, denticles 

 also occur over the surface of the body ; but in the remainder, 

 and all higher vertebrates, teeth are restricted to the mouth. 

 In addition to those on the premaxilla, maxilla, and dentary, 

 teeth may be carried by the prevomer, parasphenoid, palatine, 

 and pterygoid bones in lower vertebrates ; in the bony fish 

 teeth may even be carried on the branchial arches. In 

 Selachians, the teeth are loosely attached to the underlying 

 skeleton by connective tissue. In bony fish, they are firmly 

 fixed on to the underlying bone by " cement,'' a modified 

 form of bone, which is absorbed when the tooth is shed. In 

 some cases the teeth may be hinged. In higher forms the 

 bone grows round the base of the teeth, which thus come to 

 lie in grooves (pleurodont) or sockets (thecodont). In 

 Sphenodon and Chamaeleo the teeth are fused on to the edge 

 of the bone (acrodont condition) and are not replaced. 



The teeth of Osteolepidoti and of the earliest amphibia are 

 peculiar in that their walls are thrown into folds, giving a 

 characteristic appearance when seen in section, and which is 



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