vi EXTERNAL CHARACTERS 153 



grotesque appearance of the head of the Angler- Fish (Lophius) ; 

 the huge high trunk and abbreviated tail of the Sun -Fish 

 (Ortkagoriscus) ; and the short high trunk and long tail of 

 Notopterus (Fig. 334). 



In its external appearance the head perhaps differs more in 

 different Fishes than any other part of the body. Long and 

 flattened in the Skates and Bays, the head becomes short and 

 high in most Holocephali and in many Teleosts, or is shaped like 

 a blunt cone, as in such Dipnoi as Protopterus and Lepidosiren ; 

 or becomes long and pointed, as in the North American " Gar 

 Pike " (Lepidosteus) ; or, finally, as in the Hammer-head Shark 

 (Spliyrna), the head may be produced into great lateral exten- 

 sions, carrying the eyes at their extremities (Fig. 256, B). Apart 

 from its relative shape and size, the appearance of the head may be 

 further modified by the thinness of the investing scaleless skin, 

 which readily allows the surface and contour lines of the bones 

 of the skull to be seen through it, as in the Crossopterygii, and 

 in such Teleosts as the Siluroid genera Clarias and Callichtliys ; 

 or the skin, even if devoid of scales, may be so thick that scarcely 

 any of the bones are visible externally. The exoskeleton, whether 

 in the form of scales or bony plates, may extend to a varying 

 degree on to the surface of the head in different Teleosts, or may 

 even invest nearly the whole of the head. When, as is not infre- 

 quently the case (e.g. many Scorpaenidae) certain of the bones of 

 the skull are produced into projecting spines, the head assumes a 

 singularly formidable appearance (Fig. 424). 



The mouth differs greatly in size and position. In existing 

 Elasmobranchs it is generally crescentic in shape and always 

 ventral in position, but in certain primitive fossil members of the 

 group, as in the Palaeozoic Cladoselache, it is anterior and terminal. 

 The Sturgeon and other living Chondrostei have the mouth 

 ventral. In the Dipnoi also the mouth is ventral, but is near the 

 extremity of the snout. As a rule, the mouth is terminal or 

 nearly so in the living Crossopterygii and Holostei, and in the 

 great majority of Teleosts, although in the latter group it is 

 occasionally distinctly ventral, especially when a snout is de- 

 veloped, and it may sometimes look upwards by reason of the 

 projection of the lower jaw in front of the upper. A pronounced 

 " beak " is sometimes formed by the forward prolongation of both 

 jaws, as in the Gar Pike (Lepidosteus), with the result that the 



