Professor Owen o?i British Fotaif 'Reptiles. 93 



sttcceeiled eacli other since the earliest periods in which the 

 remains of this class can be discerned ; but the change has 

 been, upon the whole, from the complicated to the simple. 

 The Batrachian order, which is first indicated by the large and 

 powerful crocodiloid Lahyrinthodonts^ has dwindled down to 

 the diminutive and defenceless Anourans and the fish-like 

 Perennibranchians. The Saurian order was anciently repre- 

 sented by reptiles manifesting the crocodilian grade of orga- 

 nization, under a rich variety of modifications and with great 

 development of bulk and power ; it has now subsided into a 

 swarm of small Lacertians, headed by so few examples of the 

 higher or loricate species, that it is no marvel such relics of a 

 once predominating group should have found a humble place 

 in Linnaeus's catalogue of nature as co-ordinate members of 

 the genus Lacerta. 



Nevertheless, some general analogies may be traced between 

 the phenomena of the succession of reptiles as a class, and 

 those observed in the development of an individual reptile 

 from the ovum. Thus the embryonic structure of the verte- 

 brae of the existing crocodiles accords with the biconcave type ; 

 and this is exchanged, in the development of the individual as 

 in the succession of species, for the ball and socket structure 

 as the latest condition. 



The almost universal prevalence of the more or less bicon- 

 cave structure of the vertebrae of the earlier reptiles, thus esta- 

 blishes a most interesting analogy between them and the 

 earlier stages of growth of existing reptiles. 



A similar analogy has been pointed out by M. Agassiz, be- 

 tween the heterocercal fishes, which exclusively prevail in the 

 oldest fossiliferous strata, and the embryos of existing homo- 

 cereal fishes, which seem to pass through the heterocercal 

 stage. The superior number of loricate reptiles, and the more 

 complete development of the dermal armour in the crocodilian 

 genera Sleneosaurus, Teleosaurus, Goniopholis, &c., of the 

 oolitic and Wealden strata, corresponds with the prevalence of 

 the well-mailed Ganoid order of fishes in the same formations. 



The fossil reptiles, like the fossil fishes, approximate nearest 

 to existing species in the tertiary deposits, and differ from 

 them most widely in strat^i whose antiquity is highest. 



