PHYLUM CHORDATA 



245 



Scutes are present in some species, rhomboid scales in others, 

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

 cycloid scales, while rhomboid scales occur in the upper 2)art of 

 the tail. 



The Holostei 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 high and compressed ; the scales 

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

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



Fig. 902.— a Lepidotus maximus (Jurassic), s. scale ; (. 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 divei-gent forms of Teleostei which fill similar 

 positions at the present day. 



The Teleostei first appear in the Cretaceous rocks, where many 

 existing families are represented. From this period onwards the 

 three Ganoid orders undergo a progressive diminution in the 



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Q 



