344 ANNUAL REPORT SMITHSONIAN INSTITUTION, ]937 



but occasionally enter the sea voluntarily for short periods. The 

 other ^oups not specifically placed in divisions grade off into wholly 

 marine forms. Finally, there are a number of species and genera of 

 salt-water families that have taken up more or less permanent resi- 

 dence in fresh water. Most of them return to the sea to spawn. 



THE GROUPS OF FRESH-WATER FISHES 



The fresh-water fishes of the primary and secondary divisions tend 

 to group themselves on natural family lines. In other words, all or 

 nearly all species of one family usually show a similar tolerance to salt 

 water. This, together with the fossil evidence available, leads us to 

 believe that most of the famifies of the primary di\asion have carried 

 down their physiological inability to survive in the sea, as family 

 characters, from early times and probably since the origin of the groups 

 concerned. 



The fresh-water fishes of the primary division are of diverse relation- 

 ships among fishes generally. A few of them are small, rehct groups 

 of primitive organization, such as the two living families of lung 

 fishes (Ceratodontidae, with one surviving species in Australia, and 

 Lepidosirenidae, with three surWving species of the genus Protopterus 

 in Africa and one of the genus Lepidosiren in South America). Of 

 similar relict distribution are the two living genera of the primitive 

 paddle-fishes, Psephurus in the Yangtse River in China and Polyodon 

 in the Mississippi. Higher on the scale of fish evolution are the 

 paleoniscidlike bichirs of Africa and the family of bowfins, with one 

 living North American species. Somewhat similar to these last are 

 the "ganoid" garpikcs of North America, a family which I place, 

 somewhat hesitantly, in the secondary division of fresh-water fishes. 



The bulk of living sea fishes are of more speciaUzed organization 

 than these primitive families, and belong to the great subclass of 

 bony fishes, or Teleostei, and this is true of the fresh-water families as 

 well. The lowest order of teleosts is that of the herringlike fishes 

 (Isospondyli) and to it belong several families of my primary fresh- 

 water division; among them the hiodonts or mooneyes of the Missis- 

 sippi; the strange phractolaemids, pantodonts, kneriids,' and mormy- 

 rids of Africa; and the ancient osteoglossids, which were probably 

 well-nigh cosmopolitan in Eocene fresh waters. 



More than half of the true fresh-water fishes of the world belong to 

 a single order, the Ostariophysi,* distinguished from the herrings by 

 the pecuhar chain of Weberian ossicles connecting the air-bladder with 



• The Nilotic Cromerlidae seem to be larval Knerlidae. 



« See Regan (1922). A few characins, carps, and fresh-water catflshos enter brackish water In estuaries, 

 but none is known to be able to survive in the sea for more than a few hours; none could breed there. The 

 only exception of which I know Is the carp Acahara hakonensii, which Prof. J. O. Snyder tells me is occa- 

 sionally taken in the open sea off southern Japan. 



