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 Elasmobranchs. 
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 Cochlodontidae. By a somewhat similar 
process of concrescence the anomalous composite teeth of such 
Teleosts as the Diodons and Tetrodons, and of the Parrot-Fish 
(Scarus), have been evolved. The singular dental structures 
of the Holocephali are probably composite teeth, and it 1s 
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 (Aaia), 
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 
