SHARKS AND RAYS. 415 
agents to the types of invertebrate life which then became 
extinct, partly through their means. These and ganoid fishes 
having thus accomplished their work were replaced in the 
later ages by more highly elaborated and specialized forms, 
a.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 
members of their own class. Hence their own forms are 
gigantic, 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- 
drical, 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 portion 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 sharks 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 Vertebrates, 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 ag 
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 Chimera, to a well-marked 
