THE TEETH OF FISHES. 237 



dosteus (American garpike, a ganoid fish) has been figured at 

 page 78. 



Enamel is often present in a very thin layer, glazing the 

 exterior of the dentine (see Fig. 48) ; sometimes it forms a 

 mere tip, a sort of spear-point to the tooth as in the Eel and 

 the Hake (see Figs. 87 and 90), and sometimes it is very 

 thick, and itself permeated by systems of tubes (see Fig. 24). 



Cementum is of comparatively rare occurrence in fish. 



Professor Kolliker has shown that in a very large number 

 of fishes the skeleton more nearly resembles dentine than 

 true bone in its structure ; whilst the dermal scales and 

 protective spines of fish are often made up of a tissue much 

 resembling dentine (cf. Professor Williamson, Philos. Trans. 

 1849). We may say, then, that just as in the external skin, 

 bony or dentinal plates are developed for the purpose of 

 protecting it from destruction by attrition, so for a similar 

 purpose teeth are developed in that portion of the mucous 

 membrane which covers the jaws. 



Near the borderland between fish and amphibia is the 

 Lepidosiren, or Mud-fish, which is a fish rather than an 

 amphibian. The armature of its mouth is peculiar, the 

 margins of the lower jaws being formed by dental plates 

 anchylosed to the bone. These plates have upon their 

 edges five deep angular notches, the prominence of the 

 upper plate corresponding to the notches of the lower ; 

 and the edge is kept somewhat sharp by the front surface 

 being formed of very dense hard dentine, while the bulk 

 of the tooth is permeated by large medullary canals, which 

 render it softer. The cutting plates of the upper jaw are 

 developed in the median line of the palate, and there are 

 in front of them conical piercing teeth upon that forward 

 prolongation of the cartilage which takes the place of a 

 distinct vomer; these have sometimes been described as 

 being upon the nasal bone. 



It would seern that the two conical piercing teeth serve 



