XllI 



TMIVTJ'^r CIIOKDATA 



245 



Scutes are present iu some species, rliomboid scales in others, 

 and in one genus the greater part of tlie body is covered by 

 cycloid scales, Avhile rhomboid scales occur in thu upper part of 

 the tail. 



The Holostci first make their appearance in the Triassic rocks 

 and are abundant in Secondary and Lower-Tertiary strata. They 

 also (Fig. 902) show a wide diversity in form and structure. The 

 body may be spindle-shaped or higli and compressed ; the scales 

 may be rhomboid or cycloid, or nuxy exhibit every gradation from 

 rhomboid to cycloid in passing from the trunk to the tail of one 



s 



Vui. 90'2.— A Lepidotus maximus (Jvu-assic). •«. scale ; t. teeth. B, Caturus furcatus 



(Jurassic). (From Nicholson and Lydekker.) 



and the same Fish ; the teeth may be sharp and conical, or blunt, 

 rounded, and adapted for crushing. A persistent notochord is 

 present in some species, a well-ossified vertebral column in others. 



We see, then, that all the orders of Ganoids, during the period 

 of their prime, branched out into diverse forms, adapted to 

 different environments, and often resembling, in a remarkable 

 manner, the divergent forms of Teleostei which fill similar 

 positions at the present day. 



The Teleostei first aj)pear in the Cretaceous rocks, where many 

 existing families are represented. From this period onwards the 

 three Ganoid orders undergo a progressive diminution in the 



VOL. II Q 



