VERTEBRATA. 315 
They are all marine. The largest Shark found, and there¬ 
fore the largest Fish, measured forty feet in length. 
Fig. 2SS.—Thoruback (Rada clavata). European seas. 
2. Ganoidei, distinguished by their enameled scales or 
bony plates. The endoskeleton is usually not completely 
ossified; the ventral fins are placed far back; and the 
tail is generally heterocercal. The gills are like those of 
the bony Fishes, and the air-bladder has a duct, and anay 
aid in respiration. This was one of the largest orders in 
old geological history. The few modern representatives, 
as the Sturgeon, Gar-pike, Mud (or Dog) Fish, and Polyp - 
terus , are essentially 
fresh-water. 
3. Teleostei , in¬ 
cluding all the com¬ 
mon Fishes having 
a bony endoskeleton Fig. 2S9.—Gar-pike (Lepidosteusosseus). Lake Ontario 
