274 THE STUDY OF ANIMAL LIFE CHAP. 



mydoselachus) is said to be very closely allied to types 

 which occur in the Old Red Sandstone. Allied to the 

 Elasmobranchs, but sometimes kept in a separate division, 

 are two genera, the Chimcera or King-of -the -Herrings, 

 and Callorhynchus, its relative in Southern Seas. The 

 Elasmobranchs stand somewhat apart from other fishes. 



(2) A very distinct order, IIOW T decadent, is that of the 

 Dipnoi or Double-breathers. They do not depend on 

 gills only, for the air-bladder has been turned into a 

 lung. In this and several other ways a transition from 

 Fishes to Amphibians is suggested. There are many 

 extinct representatives, but only three living genera 

 Ceratodus from Queensland rivers, Protopterus from West 

 and Tropical Africa, and Lepidosiren from the Amazons. 

 Their widely separated geographical representation shows 

 that they are relics of a once widespread stock. Other 

 illustrations of this have already been noticed, e.g. 

 Onychophora, Enteropneusts, and Lancelets. 



(3) The other fishes are grouped together as Teleo- 

 stomes. Four sets of them are now represented by (a) 

 two " living fossils ' Polypterus and Calamoichthys 

 from African rivers ; (b) the sturgeon tribe ; (c) the bony 

 pike (Lepidosteus) and Amia of North America, and (d) 

 the modern rank and file of thoroughly bony fishes or 

 Teleosteans. The first three sets are often spoken of as 

 Ganoids. Herring and salmon, cod and pike, eel and 

 minnow, flounder and plaice in fact all of the common 

 bony fishes are Teleosteans. 



The wedge-like form of most fishes is well adapted for 

 rapid swimming. Most flat fish, whether flattened from 

 above downwards like the gristly skate, or from side to 

 side like the flounders and plaice, live at the bottom ; 

 those of eel-like shape usually wallow in the sand or 

 mud ; the quaint globe-fish float passively. The chief 

 organ of locomotion is the tail ; the paired fins help to 

 raise or depress the fish, and serve as guiding oars. In 

 the climbing perch they are used in scrambling ; in the 

 flying fish they serve as parachutes during the long 

 swooping leaps. In eels and pipe-fish they are absent ; 

 in the Dipnoi they have a remarkable median axis. The 



