346 



ZOOLOGY 



shining, smooth plates covering the body. A typical 

 example is found in the living African genus Polypterus, 



^^ /& j? s& ^ 



From Perrier's "Traite de Zoologie " 

 FIG. 128. Polypterus bichir. River Nile. 



twelve species of which inhabit the rivers of that conti- 

 nent. The surface of these fishes is hard and porce- 

 lainlike, and is composed of scales of which the exposed 

 portions are diamond-shaped. On removing these 

 scales from the body, it is seen that each has a peg, 

 which fits into a socket in the scale next to it. 



We now know that the group of ganoids is artificial ; 

 that is to say, it associates together fishes which are not 

 nearly related, and keeps apart those which should be 

 more nearly associated. The Polypterus is a very old 

 type, a member of a large and once dominant group 

 called Crossopterygii, showing certain resemblances to 

 the lungfishes and the primitive amphibians. Another 

 sort of ganoid fish is the garpike of the Mississippi Val- 

 ley. This is entirely different from the Polypterus in 

 many important characters, and falls in a very distinct 

 group, but it has the characteristic rhomboidal ganoid 



Photograph from Am. Mus. Natural History 

 FIG. 129. Garpike. 



