IX DENTITION 251 



habits and in the food of different Fishes, the teeth exhibit an 

 equally striking diversity in form, size, and structure. The 

 most primitive type of tooth resembles an ordinary dermal spine, 

 and is little more than a simple pointed cone. A few Elasmo- 

 branchs and many Teleostomi possess teeth of this kind. By 

 the flattening of the cone parallel to the axis of the jaw, the 

 tooth becomes triangular, and then the margins may either 

 remain smooth and trenchant, or they may become complicated 

 by the formation of marginal serrations or of accessory basal 

 cusps, and by such modifications the characteristic teeth of 

 most Elasmobranchs are formed. The simple cone may also 

 be modified to form crushing teeth short, blunt, more or less 

 hemispherical teeth or even transformed into a mosaic of hexa- 

 gonal plates, as in the Myliobatidae amongst Ela.smobrauchs. 

 Massive, flattened, scroll-like crushing teeth are also formed by 

 the fusion of adjacent teeth, or of several successional teeth, and 

 of such composite teeth we have examples in the Heterodontidae 

 and in the Palaeozoic Cochliodontidae. By a somewhat similar 

 process of concrescence the anomalous composite teeth of such 

 Teleosts as the Diodons and Tetrodons, and of the Parrot-Fish 

 (Seams'), have been evolved. The singular dental structures 

 of the Holocephali are probably composite teeth, and it is 

 certain that the highly characteristic teeth of the Dipnoi have 

 resulted from the basal fusion of primitively distinct simple 

 conical denticles. The dentition is often heterodont. In Hetero- 

 dontus (Cestracion), for example, the anterior teeth in each jaw 

 are pointed and prehensile, while the hinder ones are scroll-like 

 and crushing. Prehensile and crushing molar-like teeth are also 

 present in such Teleosts as many of the Sparidae, and in the 

 Wolf -Fish (Anarrhichas). The existence of sexual differences 

 in the dentition is illustrated in the Skates and Rays (Eaict), 

 where teeth which are simple and pointed in the male become 

 flattened and plate-like in the female. A few Teleosts, like the 

 Syngnathidae, Cyprinidae, and some Siluridae, are entirely devoid 

 of jaw-teeth. 



In addition to jaw-teeth, many Teleosts possess pharyngeal 

 or gill-teeth, developed in connexion with the inner margins of 

 the branchial arches, to which they are usually firmly ankylosed 

 (Figs. 352, 412 and 413). As a rule "the pharyngeal denti- 

 tion is inversely proportional to the extent of tooth development 



