18 DEVELOPMENT. 



enamel-pulp. In the incisors of the sargus, the development of the 

 enamel and dentine begins simultaneously upon the contiguous sur- 

 faces, and when we observe how close and compact is the package 

 of the matrix of the tooth in the alveolar cavity of the jaw, it is 

 hardly possible to conceive how either of these substances could be 

 the product of transudation from their respective pulps. It is, how- 

 ever, easier to separate the primary layers of the enamel and dentine 

 from their respective pulps than from each other ; yet if the denuded 

 surfaces of the uncalcified portions of the pulps be examined by re- 

 flected light under a compound lens of a half-inch focus, they are seen 

 to be ragged and punctate, and evidently different from the original 

 surfaces prior to the commencement of the deposition of the calcareous 

 salts in them. The formation of the enamel resembles more closely 

 that of the dentine in the fishes cited than it does in the mammalia, 

 and the enamel contains a greater proportion of persistent animal 

 matter. 



The course of calcification of the two pulps takes opposite direc- 

 tions, and in the balistes, the process finishes by the ossification of 

 the outer layer of the capsule itself, by which both the enamelled 

 crown and the base of the tooth are coated with a thin layer of bone. 

 I have not been able to discern any radiated cells in this analogue of 

 the " crusta petrosa," or cement of the mammalian teeth. It soon 

 wears off from the crown of the extruded tooth. 



In all fishes, the teeth are shed and renewed, and this not once 

 only, as in most mammalia, but frequently and during the whole 

 life time of the animal. (1) Fishes, indeed, can hardly be said to have 

 permanent teeth. The rostral teeth of the pristis constitute, perhaps, 

 the sole exception ; and these may be regarded rather as modified 

 dermal spines. 



In all cases where the first teeth are developed in alveolar cavities, 

 the succeeding ones follow them in the vertical direction, and owe the 

 origin of their matrix to the continuation, from the mucous capsule of 

 their predecessors, of a csecal process, in which the papillary rudiment 

 of the dental pulp is developed according to the laws explained in the 



(1) In a few cases it is observed, that as the fish gets old, some of the deciduous teeth are not 

 replaced ; in old Salmoindcc, the vomer thus becomes edentulous, or nearly so. 



