244 



ZOOLOGY 



SECT. 



period, and between that period and the Cretaceous, include six 

 families and a large number of genera and species. They exhibit 

 (Fig. 900) a very considerable range of variation in external 

 and internal characters. There are usually two dorsal fins, the 

 tail may be diphycercal or heterocercal, the scales rhomboid or 

 cycloid. In some genera, also, there was a persistent notochord 

 (B. nch.), the fossils showing well-preserved neural and ha3mal 

 arches, but no signs of centra. In many cases the interspinous 



B 



FIG. 901. A, Palrconiscus macropomus (Permian) ; B, Platysomus striatus (Permian). 

 (From Nicholson and Lydekker.) 



bones or proximal pterygiophores of the dorsal fins are fused into 

 a single 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 Ghondrostei 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 re- 

 semblance to Teleostei (Fig. 901). 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). 



