OF THE BRITISH ISLANDS. 81 



are viviparous, that is, produce their young alive. The 

 fecundity of fishes is in excess of that of any other division 

 of the animal kingdom. The number of eggs contained in 

 the roe of a single Cod frequently exceeds eight or nine 

 millions, while the roe of a large Turbot weighing twenty 

 pounds was found to contain over fourteen million eggs. 

 The average number of eggs produced by a Salmon having 

 a weight of twenty pounds is twenty-seven thousand, and 

 that of a Herring from twenty to fifty thousand. 



The number of known species of fish distributed through- 

 out the salt and fresh waters of the globe falls but little 

 short of nine thousand, out of which as many as two 

 hundied and thirty-two are included in the fish-fauna of 



FIG. I. AUSTRALIAN MUD-FISH (Ceratodus Forsteri). 



the British Islands. Of these, some twenty-eight or thirty 

 are inhabitants of purely fresh water, twelve or thirteen are 

 " anadromous," migrating periodically from salt to fresh 

 water or the converse, while the remainder are exclusively 

 marine forms. The fish class as a whole is sub-divided by 

 our highest authority (see Professor Huxley's 'Anatomy 

 of Vertebrated Animals ') into as many as six leading 

 sections or orders. These, commencing with the most 

 highly organised, and descending to the lowest or least 

 specialised group, take the following sequence : I. The 

 Dipnoi or Mud-fishes ; II. The Teleostei or ordinary bony 

 fishes ; III. The Ganoidei or Sturgeon tribe ; IV. The Elas- 

 mobranchii, including the Sharks and Rays ; V. The 

 VOL. i. H. G 



