522 Evolution and Distribution of Fishes 



nents yielded few ancestral isospondylous ganoid types, 

 which are evidently Old World in origin. 



The probable ganoid forerunners of the earliest teleosts 

 are Leptolepis and Thrissops, both abundant freshwater 

 fishes from Upper Lias on to Cretaceous days. These also 

 are close allies of Chirocentrites and Spathodactylus of 

 European Lower Cretaceous beds. But Chirocentrids must 

 early have reached a marine environment, and then spread 

 abroad widely, for such bulky Upper Cretaceous marine 

 genera as Portheus, Ichthyodectes and Saurodon are known 

 from Europe, America, and even Australia. 



The relation between freshwater and marine teleosts 

 is then discussed, and the presence by migration of masses 

 of the former in Cretaceo-Eocene Lake-beds of North 

 America is explained. So in early Eocene time we can look 

 to the North American continent as a great centre for evo- 

 lution of new species and even groups. 



The family Characinidae with its offshoot Gymnotidae, 

 also the Cyprinidae, the Siluridae, and Loricaridae are then 

 studied as to their evolution and distribution in time and 

 space. Tetragonopteriis is viewed as the earliest known 

 fossil representative of the first, Amyzon and Leuciscus of 

 the second or Cyprinidae. This family now comprises up- 

 ward of 200 genera and 2000 species of the North Temper- 

 ate zone, and so, "is the largest of all the families of fishes." 

 From earliest origin in the western lake area of North 

 America, during late Cretaceous time, they seem to have 

 spread into Europe during the Eocene period, also north- 

 eastward into Asia about the same time. Connection of S. 

 Europe with N. Africa during this or Oligocene time, per- 

 mitted passage also into N. Africa. Such distribution for 

 Cyprinidae is amply verified by like parallel distribution 

 for other freshwater teleosts. The family seems to have 

 attained its climax of development and abundance in the 

 Miocene age, and in such genera as Tinea, Leuciscus, 

 Rhodeus, etc. 



The sub-group Homalopteridae, like numerous other 

 cited cases, is now found in mountain streams of India to 

 Borneo, owing doubtless to diastrophic elevation of land 

 as well as of lakes in which the fishes were, during Oligo- 



