378 ANATOMY OF VERTEBRATES. 



straightcr and more parallel course, usually at right angles to the 

 outer surface of the dentine. Those conical teeth which, when 

 fully formed, consist wholly or in great part of osteo-dentine or 

 vaso-dentine, always first appear with an apex of hard or true 

 dentine. In some Fishes the simple central basal pulp-cavity of 

 such teeth, instead of breaking up into irregular or parallel 

 canals, sends out a series of vertical plates from its periphery, 

 which, when calcified, give a fluted character to the base of the 

 tooth, e. g. in Lepidosteus oxyurus. 1 This is the first step in the 

 pattern of complication which attains its maximum in Labyrinth- 

 odonts and Dendrodonts, figs. 244, 246. 



Thus, with reference to the main tissue of tooth, we find not 

 fewer than six leading modifications in Fishes : hard or true 

 dentine (Sparoids, Labroids, Lophius, Balistes, Pycuodonts, 

 Prionodon, Sphyrcena, Megalichthys, Rhizodus, Diodon, Scants), 

 osteo-dentine ( Cestracion, Acrodus, Lepidosiren, Ctenodus, Hybodus, 

 Percoids, Scicenoids, Cottoids, Gobioids, and many others), vaso- 

 dentine (Psammodus, Chim&roids, Pristis, Myliobates), plici-dentine 

 (Lophius, Holopty chius, Lepidosteus oxyurus, at the base of the 

 teeth), labyrintho-dentine (Lepidosteus platyrhinus, Bothriolepis), 

 and dendro- dentine (Dendrodus) ; besides the compound teeth of 

 the Scarus and Diodon. 



One structural modification may prevail in some teeth, another 

 in other teeth, of the same fish ; and two or more modifications 

 may be present in the same tooth, arising from changes in the 

 process of calcification and a persistency of portions or processes 

 of the primitive vascular pulp or matrix of the dentine. 



The dense covering of the beak-like jaws of the Parrot-fishes 

 (Scarus, figs. 258, 259) consists of a stratum of prismatic denticles, 

 standing almost vertically to the external surface of the jaw-bone. 

 It is peculiarly adapted to the habits and exigences of a tribe of 

 Fishes which browse upon the lithophytes that clothe, as with a 

 richly tinted carpet, the bottom of the sea, just as the Ruminant 

 quadrupeds crop the herbage of the dry land. 



The irritable bodies of the gelatinous polypes which constitute 

 the food of these Fishes retract, when touched, into their star- 

 shaped stony shells, and the Scari consequently require a dental 

 apparatus strong enough to break off or scoop out these calcareous 

 recesses. The jaws are, therefore, prominent, short, and stout, 

 and the exposed portions of the premaxillaries and premandibulars 



' Wyman, American Journal of Natural Sciences, Oct. 1843. Cuvier has given 

 an accurate view of the plaited structure of the base of the Wolf-rish's teeth in pi. 32, 

 fig. 7, of his Lemons d' Anatomic Comparee, 1805. 



