XIII 



PHYLUM CHORDATA 



227 



and it is not until the Cretaceous period that the Teleostei, the 

 present dominant order, make their appearance. From the Cre- 

 taceous onwards the Ganoids undergo a progressive diminution 

 in numbers, genus after genus and family after family becoming 

 extinct, while a corresponding 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. 850) a very considerable range of variation in external 



FIG. 850. A, restoration of Glyptolepis (Devonian) ; B, M acropoma mantelli (Cretaceous). 

 a. U. ossified air-bladder ; d.f.l, d.f. 3, dorsal fins ; h. a. haemal arches ; jug.pl. jugular plates ; 

 n. a. neural arches ; nch. position of notochord ; pet. f. pectoral fin ; pv. J. pelvic fin ; v. f. 

 ventral fin.. (From Nicholson and Lydekker.) 



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 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 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 Chonclrostei are also largely represented, from the Devonian 

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



Q 2 



