TELEOSTEI. 



509 



The fossil Ganoids appear in the Sihirian about the same time as the 

 Elasmobranchs, they are abundant from the Devonian to the Upper 

 Cretaceous when the Teleosteans begin to liecome numerous. It is very 

 doubtful whether the primitive armoured fishes [Treinataspis, Pterasfiis, 

 Cefkalaspis, Pla-ichthys, &c,) have any claim to be considered as Ganoids 

 at all. They constitute the group of Ostracodermi, which, commencing 

 in the Upper Silurian, seems to have become extinct at the conclusion 

 of the Devonian era. 



Fishes allied to the Ganoids of the present day appear in the Middle 

 Devonian, and are found in abundance until the close of the Jurassic 

 era, when they gave way to the more specialised Teleostei. In Devonian 

 and Carboniferous rocks these Ganoids may be classed in two series : — 

 Crossopterygii (Holoptychiidfe, Rhizodontida:, Osteolepidte, Coelacan- 

 thidai), allied to the living Polypterus, and the Acipenseroidei (Palee- 

 oniscida:), allied to the Sturgeons. But already in the Permian era we 

 begin to find representatives of that great semi-heterocercal series, 

 which is represented at the present day by Lepidosteus and Amia, and 

 which, in reality, passes gradually into the Physostomous Teleostei. 

 These, represented by such forms as Lepidotus^ Dapedhis^ En-gnatluts, 



Fig. 172. — Pterichthys Milleri. (Lateral View, 

 by Traquair). 



Restored 



&c., become very abundant in Jurassic rocks, while the Crossopterygii 

 and Acipenseroidei dwindle away. So does the Lepidosteid series in 

 the Cretaceous era, and in Tertiary times the Ganoids were, as now, 

 nearly a thing of the past. 



Order III. Teleostei — the " Bony Fishes." 



This order includes most of the fishes now ahve. 

 Though comparatively modern fishes, they are older than 

 was formerly supposed, as several Jurassic genera ( Thrissops, 

 Leptolepis, &c.), which used to be classed as Ganoids, must 

 be considered as actual Clupeoids, or Herring-like Teleostei. 

 It is, however, not until the Upper Cretaceous and Tertiary 



