228 



ZOOLOGY 



SECT. 



apart from the heterocercal tail, have a strong external re- 

 semblance to Teleostei (Fig. 851). Some have the characteristic 

 spindle-form of strong-swimming Fishes (A), others the high 

 compressed form of such shore-fishes as the Reef-fishes (B). 

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

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



B 



FIG. 851. A Palseoniscus macropomus (Permian); H.Platysomus striatus (Permian). 

 (From Nicholson and Lydekker.) 



cycloid scales, while rhomboid scales occur in the upper. part 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. 852) 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 

 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. 



