34 



reduce to a pulp those calcareous habitations. The gelatinous matter of the polypes is dis- 

 solved and digested, but their calcareous parts are cast out, in the condition of nearly pure 

 chalk, with which the intestines of the Parrot-fishes are found laden ; whence these fishes 

 ' have been classed amongst the geological agents by which the skeletons of the lithophytes 

 are converted into chalk *. 



Hunterian. 



124. The premandibular and premaxillary bones of the Scarus rivulatus, Cuv. et Val. 



The exposed crowns of the denticles in these jaws are not so distinctly tuber- 

 culate as in the preceding species. Hunterian. 



125. The premaxillaiy and premandibular bones of apparently the same species of 



Scarus. 



126. The maxillary, premaxillary and premandibular bones of apparently the same 



species of Scarus. The bases of the dentigerous portions of the jaws present 

 a green colour. Hunterian. 



127. The premaxillary and premandibular bones of a species of Scarus. 



Hunterian. 



128. The premaxillary and premandibular bones of a small species of Scarus. The 



symphysial surfaces of the lower jaw present a series of parallel oblique ridges. 



Hunterian. 



129. The premaxillary and premandibular bones of a species of Scarus. The sym- 



physial ridges decrease in size more rapidly towards the summit of the joint 

 than in the preceding specimen. Hunterian. 



130. The superior and inferior pharyngeal bones and teeth of a species of Scarus. 



The superior dentigerous pharyngeals present the form of an elongated, vertical, inequi- 

 lateral triangular plate : the upper and posterior margin is sharp and concave ; 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 



* Darwin, Narrative of the Voyage of H.M.S. 'Adventure' and 'Beagle,' vol. iii. p. 553. 



