SCAROIDS. 119 



This substance (PI. 52, c.) is as thick as the dentine and consists 

 of a similar combination of minute tubes and a clear connecting 

 substance. The tubes may be described as commencing from the 

 peripheral surface of the tooth, to which they stand at right angles, 

 and, having proceeded parallel to each other half way towards the 

 dentine, they then begin to divide and subdivide, the branches 

 crossing each other obUquely, and finally terminating in the cellular 

 boundary between the enamel and dentine. 



The teeth, which present this complex structure, are successively 

 developed at one extremity of the bone, in proportion as they are 

 worn away at the other; not, however, as Cuvier describes, from behind 

 forwards, in both upper and lower pharyngeal bones, (1) but in opposite 

 directions in the opposite bones, the course of succession being from 

 before backwards in the upper, and from behind forwards in the lower 

 pharyngeal bones. In the progress of the attrition to which they 

 are subjected, the thin coat of cement resulting from the ossifi- 

 cation of the capsule is first removed from the apex of the tooth, 

 then the enamel (c) constituting that apex, next the dentine (&), and 

 finally the coarse cellular central bone (a), supporting the hollow- 

 wedge-shaped tooth, and thus is produced a triturating surface of 

 four different substances of different degrees of density. The enamel, 

 being the densest element, appears in the form of elliptical transverse 

 ridges, inclosing the dentine and central bone, and external to the 

 enamel is the cement which binds together the different denticles. 



The Comparative Anatomist, conversant with the modifications 

 of the dental system in the mammiferous class, cannot bat be struck 

 with the close analogy between the adherent pharyngeal denticles 

 of the Scarus and the complicated grinders of the elephant, both in 

 form, structure, and in the reproduction of the component denticles 

 in horizontal succession. But in the fish, the complexity of the 

 triturating surface is greater than in the mammal, since, from the 

 mode in which the wedge-shaped denticles of the Scarus are im- 

 planted upon, and anchylosed to, the processes of the supporting bone, 

 this likewise enters into the formation of the masticatory surface when 

 the tooth is worn down to a certain point. 



(1) Histoire Naturelle des Poissons, torn, xiv, p. 116. 



