ORDERS OF FISHES. 



d 



221 



Fig. 107.— Ganoid Fishbs. A, rolyptefm, a living Ganoid. B, OsUoUpis, a fossil Ganoid 

 (restored) : a Fectoral fin ; 6 Ventral fin ; c Anal fln ; dd' Dorsal fins. 



are stated to possess external gills when young, which they 

 lose when grown up, thus making an approach to the Amr 

 phibia. Many of the fossil Ganoids are more or less closely 

 allied to the living Lepidosteus and Polypterus. 



Another great group of the Ganoid fishes is represented by 

 the Sturgeons {Sturioniam), in which the skeleton is always 

 very imperfectly ossified, and the head, with more or less of 

 the body, is protected by large ganoid plates, which are often 

 united together at their edges by sutures. The true Sturgeons 

 are chiefly found in the North Sea, the Caspian, and the Black 

 Sea, and they are captured when ascending the great rivers 

 for the purpose of spawning. The swim-bladder of the Stur- 

 geons is one of the chief sources from which isinglass is pre- 

 pared, and the roe is sold as a delicacy under the name of 

 caviare. The place of the Sturgeons in North America is 

 taken by the Paddle-fishes {Spatularia). 



The group of Ganoids represented at the present day by 

 the Sturgeons and Paddle-fishes was formerly represented by 

 numerous remarkable fishes, which are most abundant in the 

 system of rocks known to geologists as the " Old Red Sand- 

 stone." The graphic descriptions of Hugh Miller have placed 

 many of these fishes before us as living pictures, but space 

 will not allow of any further notice of them here. One, how- 

 ever, of the more striking forms is figured hereafter (Fig. 108). 



Order V. Elasmobranchii (Gr. elasma, a thin plate ; and 

 bragchia, gills). — This order includes the Sharks and Rays, 



