238 



ZOOLOGY 



SECT. 



basal bone. All agree in the possession of lobed fins ; the basal 

 lobe is sometimes so long as to approach the type of structure we 

 shall find to characterise the Dipnoi (vide infra). 



The Chrondrostei are also largely represented, from the Devonian 

 upwards, and include a great variety of forms, many of which, 

 apart from the heterocercal tail, have a strong external resemblance 

 to Teleostei (Fig. 912). 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 



FIG. 913. A, Lepidotus maximus (Jurassic). . scale ; t. teeth. B, Caturus furcatus 

 (Jurassic). (From Nicholson and Lydekker.) 



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 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. 913) 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. 



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



