VERTEBRATA. 315 



They are all marine. The largest Shark found, and there- 

 fore the largest Fish, measured forty feet in length. 



Fig. 288.— Thornback (Raia clavata). European seas. 



2. Ganoidei, distinguished by their enameled scales or 

 bony plates. The endoskeleton is usualty 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'eus osseus). Lake Ontario 



