a 



VERTEBEATA 



PHYLUM VIII 



six teeth in all, in many Sharks and Teleosteans these are numbered by 

 hundreds. 



In the young of almost all fishes the teeth are fixed to the cartilaginous 

 or bony support by fibrous connective tissue, and in sharks and in many bony 

 fishes this arrangement persists throughout life, so that after long maceration 

 the teeth can easily be stripped off their base. In bony fishes and many 

 ganoids the roots of the teeth become fused with the bone, and a socket-like 

 elevation usually grows round the teeth. Exceptionally, teeth are also im- 

 planted in distinct sockets. The successional teeth are usually developed by 

 the side of the functional teeth and continually push them out. 



With few exceptions the crown of the tooth consists of dentine, vaso- 

 dentine, and enamel, the root of vasodentine or osteodentine. The enamel 

 forms a very thin brilliant homogeneous superficial layer, as hard as glass, and 



Vertical section of a tooth of Lepi- 

 dotus (Sphaerodus) gigas, Ag. Con- 

 sisting Of dentine (D) and enamel (S). 

 Upper Jurassic. Much magnified. 



Fig. 13. 



Vertical section of part of a 

 tooth of a pike (Esox). Recent. 

 D, Dentine. S, Enamel. VD, 

 Vasodentine. W, Root of tooth. 

 Much magnified (after Sternberg). 



Fig. 14. 



Vertical section of a tooth of 

 Otodus. Eocene. D, Dentine. 

 S, Enamel. VD, Vasodentine. 

 Much magnified. 



is easily recognised in polarised light by its double refraction. Very fine 

 unbranched tubules frequently pass from the dentine into the enamel. It is 

 composed of phosphate of lime, with a little fluoride of lime, a little carbonate 

 of lime, phosphate of magnesia, and a very small quantity of organic matter. 

 The dentine or tooth substance contains no fluoride of lime but much more 

 organic matter than the enamel, is less hard, and dissolves more slowly in 

 acids. Typical dentine (Fig. 12, D) is traversed by very fine outwardly 

 branching tubules, which radiate outwards from the pulp or its branches, and 

 contain thread-like processes of the tooth cells (odontoblasts). The dentine is 

 also very frequently traversed by large anastomosing canals (Haversian canals), 

 in which blood-vessels penetrate, and the walls of which are covered with 

 odontoblasts. Fine dentine tubules then radiate in a peripheral direction 

 from these large canals. This vascular modification of tooth substance is 

 known as vasodentine (Figs. 13, 14, VD). 



