SCAROIDS. 115 



is indicated by its lighter colour in the plate, consists likewise of 

 minute fibres or tubes, which are thickest and straightest at the 

 periphery of the tooth, but bend and cross each other as they ap- 

 proach the central tubes, appearing to interlace and anastomose at 

 the boundary line. 



The outer wall of the common alveolar cavity in which the 

 denticles are developed, is much weaker than the dense and com- 

 pact inner wall ; it becomes thinner as it approaches the margin 

 of the jaws, and disappears at different distances in different 

 species of Scari, before it reaches that margin. Where it exists 

 at the base of the jaws, it is sometimes, as in Scarus muricatus, 

 (PI. 49, fig. 1), perforated by numerous small foramina, through 

 which foramina, in the recent fish, processes of the external perios- 

 teum are continued to the analogous membrane, lining the dentige- 

 rous cavity, and forming the capsule of each denticle. These 

 processes are analogous to the gubernacula of the second series of 

 teeth in the mammalia, and, like them, serve to conduct the new teeth 

 to the exterior of the jaw. The growing denticles become elongated 

 by the addition of successively calcified portions of their pulp to 

 their basal or posterior extremities ; the opposite end exerts a pro- 

 portional pressure against the circumference of the foramen, and, 

 causing its absorption, begins to protrude. The tuberculate crown of the 

 denticle is exposed about the time when its sides have become an- 

 chylosed to those of the previously protruded row. Thus from the 

 close apposition of the protruding denticles, the whole of the outer 

 parietes of their common alveolar cavity, subjected to the stim.ulus of 

 their pressure, is finally removed, and is replaced by the pavement of 

 mutually anchylosed teeth. 



In certain species of parrot-fish one or more of the denticles, at 

 the posterior part of the intermaxillary bones, are longer than the 

 rest, and their coronal extremity is produced into a sharp and 

 sometimes recurved point ; these teeth extend beyond the pavement 

 formed by the other denticles and constitute a weapon of offence, as 

 in the Scarus aiirofrenatus, Sc. vetula, Sc. quadrispinosus and the 

 species figured iri PI. 49, fig 2. 



In some other species, as the Scarus flavescens cf Bloch and 



1 2 



