VERTEBRATA. 
315 
They are all marine. The largest Shark found, and there¬ 
fore the largest Fish, measured forty feet in length. 
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 may 
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. 289.—Gar-pike (Lepidos'eusosseus). LakeOntario 
Fig. 2S8.—Thornback (Raid clavata). European seas. 
