HIGH SCHOOL ZOOLOGY. 75 



not tho case in Lepidosteus however, for here all the parts of 

 the latter are completely ossified, the vertebroe being in fact 

 more so than in other fishes, for their bodies are joined together 

 by a ball and socket joint, (the socket behind — opisthocoelous) 

 instead of having a cup at either end as in Amia and the Teleosts. 

 The commonest species is L. osseus, but this is replaced in the 

 Southern States by a larger form which reaches a length of 

 eight or ten feet and is known as the Alligator gar. 



26. Superficially veiy unlike the garpike, Amia nevertheless 

 resembles it very closely in internal structure. Its snout is 

 short and rounded, the lower jaws peculiar in being separated 

 by a flat skin bone, the jugular plate, but otherwise the skeleton 

 of the head is very similar to the gai'i^ike's. 



The dorsal fin is long and low, whereas in the garpike, it is very 

 far back and short and high. The caudal fin is not so unequally 

 divided, and it is marked out in the male by an eyelike spot 

 which stands out against the general dark greea hue. There 

 is only one species of Amia, A. caloa ; it is known in different 

 localities by different popular names, among which Mud-fish 

 and Lake Dog-fish are the commonest. 



27. In addition to the Ganoid genera already enumerated 

 there are two other living forms confined to the rivers of Africa. 

 These have scales like the garpike and gular plates like the 

 Dog-fish, but their paired fins differ in structure, being composed 

 of a disk-like part containing the skeleton surrounded by a 

 fringe. The commonest species is the Polypterus bichir of the 



Vv^. ?i\. — T'lihiiitcrHxhtcltir. ,'5. , 



(After Claus.) 



Upper Nile (Fig. 51) the generic name of which refers to the 

 division of the dorsal fin into a series of finlets. 



