xin 



PHYLUM CHORDATA 



237 



members of a group by the higher or more specialised forms. 

 During the whole of the Palaeozoic and the greater part of the 

 Mesozoic era the three orders of Ganoids, to-day small and isolated 

 groups, formed the whole of the Teleostomian fauna, and it is not 

 until the Cretaceous period that the Teleostei, the present dominant 

 order, make their appearance. From the Cretaceous onwards the 

 Ganoids undergo a progressive diminution in numbers, genus after 

 genus and family after family becoming extinct, while a corre- 

 sponding increase takes place in all the sub-orders of Teleostei. 



The Crossopterygii make their first appearance in the Devonian 

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

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

 (Fig. 911) a very considerable range of variation in external and 



B 



FIG. 912. A, Palaeoniscus macropomus (Permian); />. Flatysomus striatus 



(Permian). (From Nicholson and Lydekker.) 



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 haemal arches, but 

 no signs of centra. In many cases the interspinous bones or 

 proximal pterygiophores of the dorsal fins are fused into a single 



