42 8 Evolution and Distribution of Fishes 



The second of these, though now reduced to four living 

 genera, must once have been an important and widely ex- 

 tended group. For in addition to the area now inhabited 

 by the living types, fossil remains of Dapedoglossus from 

 the Green River Shales of Lower Eocene age have been 

 described by Cope, and of Brychaetus from the London 

 Clay of about the same age in S. England by Agassiz and 

 others. So if we combine our knowledge of all of these, 

 it can be said that from Wyoming southward to N. Central 

 Brazil, and thence eastward to Central Africa, again from 

 Borneo-Sumatra southeastward to Northern Australia the 

 group extended. 



But how are the wide separating gaps between Wyom- 

 ing and central Brazil, between the latter and the West 

 African continent, between Central Africa — across which 

 Heterotis extends — and Sumatra or Australia to be ac- 

 counted for? In reply it can be said that of all known types 

 — living or fossil — one that would combine somewhat the 

 characters of Dapedoglossus and Osteoglossum with tend- 

 ency toward Seleropages of Australia would most nearly 

 fulfill requirements. If then in mid or late Cretaceous 

 time such a common primitive type lived in northern S. 

 America, this spreading northward by the Isthmus bridge 

 might by shortening and condensation of body and of 

 vertebrae, give rise to Dapedoglossus of Wyoming strata. 

 But again extending its territory southeastward it probably 

 developed primitive members of Osteoglossum and Ara- 

 paima in the Guianas, in Brazil, and extending by the S. 

 Atlantis bridge reached the central and south-central Afri- 

 can areas. During passage across these areas proenvironal 

 adaptations in some individuals resulted in evolution of 

 Heterotis which spread gradually during Eocene to Mio- 

 cene time from West Africa to the White Nile. It seems 

 then to have been blocked in its eastward extension by the 

 eastern walls of the Nile and the Red Sea rift-valleys. A 

 series that more nearly retained primitive characters of 

 Osteoglossum, seems to have passed southeastward by 

 Madagascar and the Afro-Indian bridge, till It reached the 

 East Indian region. Here, though denudation, earth- 

 shrinkage, volcanic action, and land displacements have 



