250 



FISHES. 



mopolitan gro^^p of Percina. Biplomystax, an Arioid fish of 

 Chili, and Ncmatogcnys seem to have crossed the Andes from 

 Tropical America at a comparatively early period, as these 

 genera are not represented on the eastern side of South 

 America ; the Tricliomyderina occur on both sides of the 

 Andes, which they ascend to a considerable height. Rdro- 

 imma is a true Salmonoid, allied to, and representing in the 

 Southern Hemisphere the Northern Smelt, Osmerus. In 

 both these genera a part of the specimens live in the sea, 

 and ascend rivers periodically to spawn ; another part remain 

 in rivers and lakes, where they propagate, never descending 

 to the sea, this freshwater race being constantly smaller than 

 their marine brethren. That this small Teleostean of the 

 Northern Hemisphere should reappear, though in a generic- 

 ally modified form, in New Zealand, without having spread 

 over other parts of the Southern Zone, is one of the most 

 remarkable, and at present inexplicable facts of the geo- 

 graphical distribution of freshwater fishes. 





Fig. 104. — Ha]}lochiton zchra, Straits of Magelha^u. 



