SHARKS AND RAYS 415 



agents to the types of invertebrate life which then became 

 €xtinct, partly through their means. These and ganoid fishes 

 having thus accomplished their work were replaced in the 

 later ages by more liighly elaborated and specialized forms, 

 i.e., the bony fishes. Sharks and skates are engines of de- 

 struction, having been since their early appearance in the 

 Upper Silurian age the terror of the seas. Their entire 

 structure is such as to enable them to seize, crush, tear, and 

 rapidly digest large invertebrates, and the larger marine 

 Tnembers of their own class. Hence their own forms are 

 g;igantic, soft, not protected by scales or armor, as they have 

 in the adult form few enemies. Hence they do not need a 

 high degree of intelligence, nor special means of defence or 

 protection, though from their activity the circulatory system 

 is highly developed. 



In the general form the sharks are long and somewhat cylin- 

 drijal, with the head rather large, often pointed, sometimes, 

 in in the hammer-headed shark, extraordinarily broad, with a 

 •capacious mouth, situated in the under-side of the head. 

 The body tapers behind, and the caudal jvortion is unequally 

 lobed, the upper lobe being much longer than the lower, 

 upturned and supported by a continuation of the vertebral 

 ■column, while the tail-fins of bony fishes are equally lobed 

 and consequently called homocercal ; those of sliarks are 

 unequally lobed, and are therefore said to be heterocercal. In 

 this respect they resemble an early stage in the development 

 of bony fishes, such as the trout or herring. Sharks, like 

 bony fishes, have two pectoral and generally two ventral 

 fins ; these two pairs of fins corresponding to or homologous 

 with the limbs of air-breathing Vertelariites, and besides this 

 there is one or usually two dorsal fins, and an anal fin, the 

 latter situated behind the vent. 



The skin is either smooth or covered with minute placoid 

 scales (see Fig. 385) ; the integument of such species as 

 are provided with these fine scales forming shagreen. While 

 the spinal column is in the sharks usually cartilaginous, and 

 easily cut with a knife, there are different grades of devel- 

 opment from certain forms, as the Chimsera, to a well-marked 



