up the Rio Negro. 



137 



corresponds to cattle for the population along the 

 banks. Another problem to be solved is, how far 

 this phenomenon of the local distribution of fishes 

 is repeated in the great affluents of the Amazons." 



Referring again to this remarkable discovery 

 Agassiz says : The species which lay their eggs in 

 the sand belong to the genera Hydrogonus and 

 Chaetobranchus. Like the North American Pomotis, 

 they build a kind of flat nest in the sand or mud, in 

 which they deposit their eggs, hovering over them 

 until the young are hatched. The species which 

 carry their young in their mouth belong to several 

 genera, formerly all included under the name of 

 Geophagus by Heckel. I could not ascertain how 

 the eggs are brought into the mouth, but the change 

 must take place soon after they are laid, for I have 

 found in that position eggs in which the embryo had 

 just begun its development, as well as those in a 

 more advanced stage of growth. Occasionally, in- 

 stead of eggs, I have found the cavity of the gills, 

 as also the space enclosed by the branchiostegal 

 membrane, filled with a brood of young already 

 hatched. The eggs before hatching are always 

 found in the same part of the mouth, namely, in the 

 upper part of the branchial arches, protected or held 

 together by a special lobe or valve formed of the 

 upper pharyngeals. The cavity thus occupied by 

 the eggs corresponds exactly to the labyrinth of that 

 curious family of fishes inhabiting the East Indian 

 Ocean, called Labyrinthici by Cuvier. This cir- 

 cumstance induces me to believe that the branchial 

 labyrinth of the eastern fishes may be a breeding 



