2 THE FRESHAVATEK FISHES OF EUROPE. 



fishes in our rivers, because they exemplify one of the ways in which fresh- 

 water species of marine genera origiuate. 



Groups of Fresh-wateii Fishes. 



Fresh-water Fishes may be referred to five of the great groups into wliich 

 the Fish class is divided. 



The spiny-finned order, named Acanthopterygii, is represented by the Perch 

 family, the Cottidae or Bull-heads, the Goby family, the Blennies, and the 

 Sticklebacks, while the Grey Mullets and ittherines may be associated with 

 them. 



The second order, termed Pharyngognathi, in which the fins are spiny 

 and the lower pharyngeal bones united together, has no representative in the 

 fresh waters of Europe. 



The third order, Anacanthini, in which there are no bony rays in the 

 vertical fins, has but one representative in the Cod family, known as the 

 Burbot ; while the Pleuronectidte or Flat Fishes are represented by the 

 Flounder, Plaice, and Sole, which are otherwise marine. 



The fourth order, Physostomi, has the air-bladder opening into the throat 

 by a duct, so that the fishes are easily able to adapt themselves to varying pres- 

 sure in the water; and this persistence of the pneumatic duct probably accounts 

 for the circumstance that by far the larger number of fresh-water fishes are 

 referable to this order. Among them are representatives in the fresh waters 

 of Europe of eight families, the Siluridse, represented by the Wels, the 

 Cyprinidae, which includes the Carp tribe. Barbels, Gudgeons, Roach, Dace, 

 Rudd, Minnow, Tench, Bream, and a miiltitude of others whose Continental 

 names have yet to become familiar. The Clupeidse or Herrings are repre- 

 sented by species of Shad and the Whitebait. The Salmon idee are known 

 to us from Salmon, Trout and Charr, not to mention the various species 

 of Coregonus, the Smelt, and the Grayling. The Pike is the only represen- 

 tative of the Esocidae. Umbra represents the Umbridaj ; the Cyprinodontidse 

 have some Continental representatives; and there are in Europe two or three 

 species of Eels in the family Anguillidae. 



The Ganoid order of fishes is represented by the Sturgeons, which form 

 the family Acipenseridie. 



And the remarkable group of Lampreys, which are comprised in the order 

 Cyclostomata, constitute the family Petromyzontidae. 



Classfkication. 

 The dilTerences of structure which characterise the types of fishes here 

 enumerated excmiiliry llic varia])ility of lish organisation. The nature of the 



