TEETH OF FISHES. 379 



are encased by the above-described complicated dental covering. 

 The polypes and their cells are reduced to a pulp by the action of 

 the pharyngeal jaws and teeth, that close the posterior aperture of 

 the mouth. The superior dentigerous pharyngeals, fig. 255, present 

 the form of an elongated, vertical, inequilateral, triangular plate : 

 the upper and anterior margin forms a thickened articular surface, 

 convex from side to side, and playing in a corresponding groove or 

 concavity upon the base of the skull ; the inferior boundary of the 

 triangle is the longest, and also the broadest ; it is convex in the 

 antero-posterior direction, and flat from side to side. On this 

 surface the teeth are implanted, and in most species they form 

 two rows : the outer one consisting of very small, the inner one 

 of large, dental plates, which are set nearly transversely across 

 the lower surface of the upper pharyngeal bones and teeth, 

 in close apposition, one behind the other : their internal angles 

 are produced beyond the margin of the bone, and interlock with 

 those of the adjoining bone when the pharyngeals are in 

 their natural position ; the smaller denticles of the outer row 

 are set in the external interspaces of those of the inner row. 

 The single inferior pharyngeal bone consists principally of an 

 oblong dentigerous plate, 1 supported by a strong, slightly curved, 

 transverse, osseous bar, the extremities of which expand into thick 

 obtuse processes for the implantation of the triturating muscles. 

 A longitudinal row of small oval teeth alternating with the 

 large lamelliform teeth, like those of the superior pharyngeals, 

 bounds the dentigerous plate on each side ; the intermediate space 

 is occupied exclusively by the larger wedge-shaped teeth, set 

 vertically in the bone, and arranged transversely in alternate and 

 pretty close-set series. 



The dental plates are developed in wide and deep cavities in 

 the substance of the posterior part of the lower, and of the ante- 

 rior part of the upper pharyngeal bones. The teeth exhibit 

 progressive stages of formation as they approach those in use ; and, 

 as their formation advances to completion they become soldered 

 together by ossification of their respective capsules into one com- 

 pound tooth, which soon becomes anchylosed by ossification of the 

 dentinal pulp to the pharyngeal bone itself. 



In the dentine of the pharyngeal teeth of the Scarus the 

 dentinal tubes average a diameter of y^ M of an inch, and are 

 separated by interspaces equal to twice their own diameter. The 

 course of these tubes is shown in fig. 262, d, in which they are 



1 v. pi. 51, fig. 3. 



