ORDERS OF FISHES. 



151 



the exoskeleton is absent, and in no case is tlie mouth fur- 

 nished with teeth. The tail is heterocercal. 



This sub-order comprises the living Sturgeons (fig. 491), 

 and is not known with certainty to have come into existence 

 before the Eocene Tertiary, where it is represented by the 

 Acipenser toliapicus of the London Clay. In the Lias, how- 

 ever, occur two species of the singular genus Cliondrosteus, 

 which have usually been referred 

 here, and have been regarded as 

 being most nearly allied to the 

 Paddle -fishes iS'patularicC) of North 

 America. The skull, however, is 

 more completely ossified than is the 

 case with any living members of the 

 Sturioniclm ; and the true place of 

 Chondrostetis must be regarded as 

 uncertain. 



We may also place here, at any 

 rate provisionally, the Devonian 

 genus Macropetalichthys, species of 

 which are known to occur in both 

 the Old and New Worlds. In this 

 genus are included large fishes, in 



which the skull is protected by large polygonal ganoid plates 

 (fig. 517), the surface of which is enamelled and tuberculated. 

 The orbits are of large size, and both scales and teeth appear 

 to have been entirely wanting. 



Fig. 517. — Diagram of tlie skull 

 of Macropetalichthys SuUivanti, 

 viewed from above, and greatly 

 reduced in size. Prom the De- 

 vonian of North America. (After 

 Newberry.) 



