CHAPTER VI. 



SUB-CLASSES AND ORDERS, 



OF the more recent schemes of classification we have adopted 

 that which divides the fishes, living and extinct, into four 

 sub classes : 



1. Lung-fishes and Mud-fishes. 



2. Bony fishes and Ganoids. 



3. Chimaeroids. 



4. Sharks and Rays. 



The lung-fishes and mud-fishes (DIPNOI) have a heart with 

 three chambers, and have lungs as well as gills, and their 

 nostrils lead into the hinder part of the mouth cavity like those 

 of animals of higher organisation. Up to the present they have 

 only been found in Australia, Africa, and South America ; and 

 as they are unknown in British waters they are beyond our scope. 



With them out of the way we can describe the fishes we have 

 to deal with as cold-blooded vertebrates adapted for life in the 

 water, breathing by gills, having a heart with two chambers, a 

 mouth with distinct jaws, and limbs either absent or modified 

 into fins. They form the fifth class of the animal kingdom, 

 those that precede it being the mammals, the birds, the reptiles, 

 and the amphibians. The class can be divided into the four 

 sub-classes already mentioned, the three with which we are left 

 being : 



1. TELEOSTOMI or Bony-fishes and Ganoids. 



2. HOLOCEPHALI or Chimseroids. 



3. ELASMOBRANCHII or Sharks and Rays. 



Dealing with these in our customary way, the first sub-clasS 

 ^an be divided into orders thus : 



TELEOSTOMI : 



Paired fins fan-like ACTINOPTERYGII. 

 Paired fins lobate CROSSOPTERYGII. 



As the latter are not represented in the British fauna, and 

 have only two surviving species (the bichir and the reed-fish) we 



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