Summary of Conclusions Reached 517 



tralia, compels us to accept that after spreading over 

 African, Indian, and Australian lakes and river swamps, the 

 group gradually became restricted to Australia and Africa, 

 while from one of the African species of Protopterus that 

 had migrated westward across the Afro-American bridge 

 Lepidosiren may have evolved. Ceratodus however seems 

 to have lingered on in N. America, till at least the close of 

 the Cretaceous, if the fragmentary specimens described by 

 Cope are accepted in evidence. 



The crossopterygian ganoids are viewed as a decidedly 

 higher and more evolved group than any of those just 

 treated of, specially in their endoskeletal and cephalic com- 

 plexity, in their advancing ossification of the notochordal 

 tube, in the perfectioning and attachment of the paired and 

 unpaired fins, and in a higher type of respiratory inter- 

 change. 



They are viewed as an intermediate offshoot between 

 primitive dipnoan and pleuracanth forms that probably 

 started in late Silurian times. Their close association with 

 Palaeospondylus, with Dipneustean, and with Elasmobranch 

 species, as shown by the records of Agassiz, Flett and 

 others, in lakes of Old Red age, and their total absence 

 from marine beds, proves that all had evolved in conti- 

 nental centres. Distinct evolutionary advance is observed 

 in the genera through Mid Old Red rocks till Holoptychius, 

 Bothriolepis, and allies are met with in the Upper series. 

 But the distribution in space by that time is arresting. For 

 evidently originating in eastern Atlantis or Eria during the 

 late Silurian period, they had spread by degrees west- 

 ward to central N. America as well as southward to 

 Australia and Antarctica. But the exact line of advance 

 southward — though probably by Australia — has yet to be 

 accurately determined. 



Just as change from Lowest to Uppermost Old Red 

 beds presents totally new assemblages of species and even 

 genera of Crossopterygians, so in transition to the Carboni- 

 ferous, new and more specialized genera like Rhizodus, 

 Strepsodus and Megalichthys appear. But all of these in- 

 habited quite distinct as well as freshwater areas, compared 

 with those from which Davis, Worthen, Traquair and 



