XXll INTRODUCTION, 



This law is well exemplified in the teeth, although in the comparison 

 of these organs we are necessarily limited to the range of a single 

 primary group of animals. We have seen, for example, that the 

 dentine is scarcely distinguishable from the tissue of the skeleton in 

 the majority of fishes : but that its peculiarly dense, unvascular 

 and resisting structure, which is the exceptional condition in fishes, 

 is its prevalent character in the teeth of the higher vertebrates. 



So likewise with the enamel ; this substance, which under all 

 its conditions bears a close analogy with the dentine, is hardly distin- 

 guishable from that tissue in the teeth of many fishes(l). The fine cal- 

 cigerous tubes are present in both substances, and undergo similar sub- 

 divisions ; the directions only of the trunks and branches being re- 

 versed, agreeably with the contrary course of their respective develop- 

 ments. The proportion of animal matter is also greater in the enamel 

 of the teeth of fishes than of the higher vertebrata ; and the propor- 

 tion of the calcareous salts incorporated with the animal constituent 

 of the walls of the tubes is greater as compared with the sub-crystal- 

 line part deposited in the tubular cavities. In reptiles, the proportion 

 of the hardening salts and consequently the density of the enamel are 

 increased, but the course, size, and ramification of the calcigerous 

 tubes still bear considerable analogy to those of the dentine ; and the 

 prismatic form of the calcigerous tubes, (2) their minute striations, 

 and the superficial transverse wavy linear ridges, which constitute 

 the characteristic features of the enamel in the mammalian class, are 

 not present in that tissue in the cold-blooded vertebrates. 



The enamel is the least constant of the dental tissues : it is more 



(1) Sargus, PI. 43, fig. 2 ; Phyllodus, PI. 44, fig. 2 ; Scarus, PI. 50, PI. 52. 



(2) I apply this term to the so-called prismatic fibres of human and other mammalian 

 enamel for reasons which will appear in the sequel. 



