524 Evolution and Distribution of Fishes 



numerous marine Cretaceous to Miocene genera like Porth- 

 eus, Iclithyodectes, and more recent derivatives. 



The origin of the Clupeidae from ancestral Jurassic 

 Leptolepidae, and the gradual splitting up of the genera 

 into a freshwater series that is typified by Diplomystus, also 

 into a derivative marine group that is typified by Clupea, 

 proceeded from early Cretaceous time onward. In con- 

 trast to Osteoglossidae, but resembling Cyprinidae, the 

 Clupeidae remained largely a North American family, but 

 migrant marine outliers passed gradually down the coast 

 to S America, and later across even to India and New 

 Zealand with other groups. 



The closely allied family Salmonidae is in fullest sense 

 a freshwater and Northern Hemisphere group, that prob- 

 ably evolved from an early Eocene stage onward, and from 

 a freshwater clupeoid ancestry like or allied to Diplomystus. 



The most primitive genera structurally are freshwater 

 or anadromous, the few marine ones are the more evolved. 

 Like all primitive freshwater teleosts the air-bladder is 

 well developed. But in a few marine offshoots and in the 

 derivative marine families Gonorhynchidae, Cromeriidae, 

 Stomatidae, and Alepocephalidae, the air-bladder is rudi- 

 mentary or absorbed. 



In study of the Haplomi or Esociformes the families 

 Galaxidae and Aplochitonidae are viewed as connecting and 

 freshwater alliances that unite the families already ex- 

 amined with the more evolved teleosts. A detailed study 

 of both families, specially from the standpoint of their 

 striking distribution, is reserved to a future chapter. 



But regarding other Haplomi these are first traced in 

 early Cretaceous marine strata, and in the families Encho- 

 dontidae. Scopelidae, and Chirothricidae, whose deriva- 

 tion from more primitive and probably Jurassic freshwater 

 ganoids has still to be unravelled. But after suffering ex- 

 tensive destruction, toward the close of that period, sur- 

 viving descendants multiplied in freshwater centres as the 

 Esocldae, Dalllidae, Cyprlnodontldae, and Percopsidae, also 

 in the sea as the Scopelidae, Aleposauridae, and allies. In 

 succession the HeteromI, the Catosteomi, and AnacanthinI 

 are studied along lines that parallel those pursued above. 



