92 
ROOM XI. 
Many of them are viviparous; others produce 
eggs inclosed in a hard, transparent, yellow 
shell, elongated at the ends into filaments. 
They have been divided into many genera, ac* 
cording to the form of the nostrils, the position 
of the fins, and the absence or presence of the 
aperture behind the eyes. Some of them, as the 
Cessations and Spine Sharks, have a large bony 
spine in the front of the dorsal fin. 
The Hammer-headed Shark ( Zygcena ) is pe¬ 
culiar for the head being flattened, truncated 
in front, and dilated 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 produced into a long spike 
armed on the sides with implanted bony spines. 
This horn, whence they derive their name, is so 
powerful, that they do not fear to attack the 
largest cetaceous animals. 
The Rays ( Raidce ) are known by their flat¬ 
tened bodies, by their large fleshy and expanded 
pectoral fins, united in front to the muzzle, and 
behind to the ventral fin and the spine. Their 
eggs have a brown coriaceous shell, of a qua¬ 
drangular form, with the angles prolonged into 
points. The tail of some, as the Rhmobatis 
and 
