VERTEBKATA. 



249 



In the spring-firmed Fishes, the ventrals are generally 

 under or in front of the pectorals, and the scales are cte- 

 noid, as in the Perch, Mullet, and Mackerel. 



Order 4. Ganoids include the Sturgeons, (Fig. 147,) 



FIG. 147. The Sturgeon, {Acipenser Sturio.) 



Bony-pike, Polypterus, and many extinct forms. The 

 skeleton is rarely completely ossified ; the ventral fins 

 are placed far back, and the tail is heterocercal, or une- 

 qually lobed, from the vertebra continuing in the upper 

 lobe. 



Order 5. Elasmobranchs (clasma, a thin plate) contain 

 Sharks, Rays, and Chimeras. The gills are formed of 

 thin laminae, arranged like the leaves of a book. They 

 have a cartilaginous skeleton, and a harsh skin called 

 " shagreen." The gill-openings are uncovered, and the 

 mouth is generally under the head, (except in the Chi- 

 merae.) The ventral fins are placed far back, the pecto- 

 rals are large, and in the Rays enormously developed 

 and the tail is heterocercal. 



