254 FISHES 



of which the head region is figured seems to show most re- 

 semblance to Palaeozoic sharks. It has six gill-slits, the mouth 

 is terminal and the teeth three-pronged. 



Forms allied to the Port Jackson shark, Cestracion, have 

 been recognised in the Carboniferous formations, and Cestracion 

 itself from the middle of the Secondary period onwards. Mem- 

 bers of the Scyllium family, to which the common spotted dog- 

 fishes belong, in the middle of the secondary period, and in the 

 Cretaceous or Chalk period occurs the spiny dog-fish (Acan- 

 thias). The latest sharks to appear are the great man-eating 

 species of Carcharias and their nearest allies, remains of which 

 have only been recognised in the Tertiary formations. 



Assuming that the ancestral fishes were Elasmobranchs, as 

 we are bound to do from the fact that the flat bones of the 

 head and of the pectoral girdle of bony fishes are derived from 

 the dermal denticles of the Elasmobranch, the earliest bony 

 fishes must have had a Crossopterygian or fringed structure in 

 their lateral fins. From the Crossopterygian fringe-finned 

 type was derived the ray-finned type by the reduction of the 

 internal radials, and the lung-fish (Dipnoan) in another direc- 

 tion by the development of an elongated axis to form the 

 biserial or pinnate type of fin. The Crossopterygian is there- 

 fore a central type, and its fossil forms should be among the 

 oldest. This is, in fact, the case but they are not found among the 

 fossils at present known to distinctly precede the Dipnoi ; 

 examples of both groups occur in the Devonian age, most of 

 them having been obtained from the Old Red Sandstone of 

 Scotland. The earliest of these two types are much more 

 similar to each other than the later representatives, as is easily 

 seen by comparing the Crossopterygian Ostcolepis with the 

 Dipnoan Dipterus both from the Old Red Sandstone. The 

 chief difference between them is the presence of well-developed 

 teeth on the dermal jaw-bones of the former, their absence in 

 the latter. The edge of the upper jaw in Dipterus is, however, 

 calcified and bony, although distinct maxillae and premaxillae 

 cannot be recognised, and in this and other respects the 

 existing Dipnoi are not primitive but degenerate or modified. 

 In both forms the tail is heterocercal, and there are two dorsal 

 fins and one ventral. With regard to the scales, those of Osteo- 

 lepis are of the Ganoid type, rhomboidal and enamelled, those 



