56 NATURAL HISTORY. [UPPER FLOOR. 
and the absence or presence of the aperture behind the 
eyes. Some, as the Cestrations and Spine Sharks, have a 
large bony spine in the front of the dorsal lin. 
The Hammer-headed Shark ( Zygaena ) is peculiar for 
the head being flattened, truncated in front, and extended 
horizontally on the sides, so as to resemble a hammer. 
The Sea Angels ( Squatina ) have a depressed body, and 
the mouth placed at the end, and not beneath the muzzle. 
The Saw Fish ( Pristis ) have with the long body of the 
sharks, the branchial opening below, and the muzzle pro¬ 
duced into a long blade armed on the sides with implanted 
bony spines. This instrument, whence they derive their 
name, is so powerful, that they do not fear to attack the 
largest cetaceous animals. 
The Rays ( Raiidce ) are known by their flattened bodies, 
by their large fleshy and expanded pectoral fins, united in 
front to the muzzle, and behind to the ventral fin and the 
spine. The mouth of most of them is armed with tuber¬ 
cular teeth placed in close quincunx order on the maxillae. 
Their eggs have a brown coriaceous shell, of a qua¬ 
drangular form, with the angles prolonged into points. 
The tail of some, as the Rhinobates ( Rhinobatus ) and 
Rhine {Rhino), is thick, like those of the sharks; in 
others, as the true Ray ( Raia ), it is slender, and often 
armed by small spines. In the Sting Ray (Trygon) it is 
very long and slender, and armed with a long bony spine, 
serrated on both its edges. The teeth and caudal spines of 
these fishes are often found in a fossil state, when the 
former have been called palates. 
The Sea Eagles ( Myliobatis) have a long tail like the 
Sting Rays, but their pectoral fins are very broad, so that 
they in some measure resemble a bird of prey with its 
wings extended. The teeth of the Sea Eagle are large flat 
plates, arranged in a tessellated form. 
The Cephalopterse ( Cephaloptera ) very much resemble 
the Sea Eagles, but their head is truncated in front, and 
the anterior edge of the pectoral fin expanded like two 
horns. 
The Electric Ray ( Torpedo ) is peculiar for its fiddle¬ 
shaped body. 
The last family of fishes is that of the lampreys ( Retro - 
myzidce), whose skeleton is the most imperfect of all the 
