ORDERS OF FISHES. 



245 



external gills when young, which they lose when grown up, thus 

 making an approach to the A mphibia. Many of the fossil Ganoids 

 are more or less closely allied to the living Lepidosteus and Polypterua. 



Another great group of the Ganoid fishes is represented by the 

 Sturgeons {Acipenseridw), in which the skeleton is alway very im- 

 perfectly 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 (fig. 167, B). The true Sturgeons are 

 confined to the seas and rivers of the Northern hemisphere. They 

 mostly inhabit rivers ; but some live in salt water, and ascend rivers 

 for the purpose of spawning. They grow to a great size^ and possess 

 a toothless mouth on the under surface of the snout. A curious 

 Ganoid found in the Mississippi is the Paddle-fish (Spatularia). 



The living Ganoid fishes are essentially fresh- water types ; and 

 they are merely scattered survivors of a very extensive series of 

 foims, most of which are now extinct. 



Order V. . Elasmobranchii. 



This order includes the Sharks, Dog-fishes, Bays, &c., and is dis- 

 tinguished by the following characters : The skull and lower jaw 



rig. 176. — A, Spinax acantJtias, one of the Dog-fishes ; B, CliimmTa monsirosa. 



are well developed, but the skull is not composed of distinct bones, 

 and simply forms a kind of cartilaginous box. The vertebral col- 

 umn is cartilaginous, but usually composed of distinct vertebra;. 



