PEOPLING OF LAND AND SEA 157 



permanent inhabitants of rivers. Only one group of Sharks 

 has become lacustrine, Carcharias gangeticus ; but of the 

 three types of Ganoids still persisting, the Sturgeons, 

 Lepodisteus and Amia, only the first spawn in rivers, and the 

 other two do not leave their streams. In the same way 

 the Crossopterygians which, with the Ganoids, were the 

 most common fishes, and the most highly organized at the 

 time of the Carboniferous Formations, are represented to-day 

 only in the rivers of Africa by two closely related genera, 

 Polyptenis and Calamoichthys. The Dipnoian Fishes, which were 

 the first, nevertheless, to develop lungs, the organs of aerial 

 respiration, now exist only in three freshwater genera : 

 Proiopterus of Africa, Lepidosiren of Mexico, and Neoceratodus 

 of Australia. This geographical distribution indicates that the 

 first invasion of the fresh waters took place almost 

 simultaneously in various parts of the world during the Primary 

 Period. However, as in the case of the Molluscs, this was not 

 the only invasion that occurred. The Bony Fish, in their turn, 

 invaded the rivers shortly afterwards. One of the oldest families 

 of this group, the Siluridae, although rather poorly repre- 

 sented in Europe by two or three species, the gigantic Silurus 

 glanis of the Danube, and Silurus arisiotelis of Macedonia, is 

 yet possessed of an astonishing plasticity, and has invaded 

 almost all the rivers of the world in varied forms which have 

 subsequently been copied by all the other freshwater Fish, with 

 abdominally placed pelvic fins, retaining none the less the funda- 

 mental characters of the skeleton of its operculum. Like the 

 Rays, it produced a group of electric Fish, M alapterurus , of Africa. 

 Then followed the whole series of Fish with ventral fins far re- 

 moved from the pectoral, as in the primitive Fish, which Cuvier 

 called " les malacopterygiens abdominaux," in which the swim- 

 bladder opens into the oesophagus or stomach (the Physostomi 

 of J. Miiller) ; that is to say, Trout, Pike, the long series of 

 Cyprinidse, to which the majority of the Fish of our rivers and 

 swamps belong, Gudgeon, Barbel, Dart, Carp, Bream, Roach, 

 Tench, Loach, etc., which are represented elsewhere by the 

 Cyprinodontidse. The Herring, Sardine, and Anchovy, all 

 related to this type, have continued to live in the sea; but 

 the Shad, which belong to the same family, namely the Clupeidae, 

 come to the rivers to spawn, like the Salmon, relations of the 

 Trout. These are Fish of the same group that have furnished, 



