Geographic and Geologic Relations 423 



The close affinity between such S. American genera as 

 Anacyrtiis, Salm'mus, Xiphostoma, and Cynodon, with 

 African genera like Hydrocyon and Sarcodaces; or between 

 Piabucina and Tetragonopterus as compared with Alestes 

 and Micralestes of Africa, almost demands an Important 

 land connection between the two continents during late 

 Cretaceous and early Eocene times. But further the ex- 

 isting distribution of many species of the African genera, 

 agrees with what is now known to be true of other fresh- 

 water groups of fishes. For not merely genera, even species 

 have a continuous range from Senegal, across Central 

 Africa to the Nile Valley. The older records which demon- 

 strated this, have been abundantly confirmed and supple- 

 mented by the recent valuable work of Moore (259), 

 and the Investigations of Boulenger (227). Thus Alestes 

 macrolepidotus is continuous from the rivers of Senegal 

 across Central Africa (Lake Tanganyika) to the Nile val- 

 ley. Distichodus rostratus with Its near ally D. niloticus 

 cover the same territory, while other of the species extend 

 from Mozambique to the Lower Nile. Citharinns geofroyi 

 is found alike at Gambia and along the Nile valley. Other 

 species occur only from W. Africa to the Congo basin. 



In some cases even, incipient or recently established 

 species, due to geographic isolation, can be traced. Thus 

 while Peters reduces the African Hydrocyon to one species 

 with varieties, Boulenger is compelled to recognize three. 

 Two of these H. forskalii and H. hrevis are from the Nile, 

 H. lineatus extends from Senegal across the Congo basin 

 to Tanganyika, the Zambesi, and even to the Limpopo river 

 In the Transvaal. But, as Boulenger has shown, not a few 

 of the African Characinidae are peculiar to the Congo, or 

 to the "Rift-valley" Lakes, and particularly to Tanganyika. 

 Thus of Petershis that author says It Is "very near to 

 Tetragonopterus of which the very numerous species inhabit 

 Central and South America. It differs in the complete 

 absence of teeth on the maxillary bone, as In Alestes and 

 Micralestes, from which it is probably derived." The four 

 species that he describes are all peculiar to the Congo basin. 

 All that Is necessary then, for explanation of the Afri- 

 can species, is the migration eastward Into Africa across 



