MESOZOIC ERA: THE AGE OF REPTILES 



535 



The lungfish almost disappeared from the seas of the Mesozoic, but 

 a few (the best known of which is Ceratodus) have succeeded in living 

 on in small numbers to the present. 



Ganoids (Fig. 497 B, C) were the common 'fish of the Tnassic and 

 Jurassic. Although they had no bony skeleton, they are well pre- 

 served because of their thick, 

 enameled scales which were excep- 

 tionally well suited for fossiliza- 

 tion. The ganoids are, as a rule, 

 rather small fish and never attained 

 the size of sharks. As the modern 

 fishes with bony skeletons (teleosts) 

 increased during the Cretaceous 

 and Tertiary, the ganoids gradu- 

 ally disappeared until, at present, 

 only a few species are in existence. 

 Two of the commonest living 

 ganoids are the gar pike and the 

 sturgeon, both of which are ex- 

 tremely plentiful in some localities. 



The bony fishes (teleosts), de- 

 scendants of the ganoids, have 

 been found in small numbers in 

 the Lower Jurassic, but probably 

 began in the Triassic. They held 

 a subordinate place, however, until 

 the Cretaceous, when they ap- 

 peared in great numbers. Among 

 the teleosts of the Cretaceous were 

 herring, cod, salmon, mullet, perch, 

 and catfish. One characteristic 

 Cretaceous type, Portheus (Fig. 

 497 D), should be mentioned. It 

 was a teleost that occasionally at- 

 tained a length of fifteen feet and 

 was provided with large, flattened, 

 irregular teeth. The suddenness of the appearance of teleosts was 

 due to the fact that, once they were established and able to compete 

 with the fish of that time, there was no hindrance to their migration, 

 and, in a comparatively few years, geologically, they had spread into 



Fig. 498. — An ancestral skate, 

 Rhinobatus (Jurassic). 



