TRANSACTIONS OF THE SECTIONS. ifO 



cies, and thence through the gradually disappearing concavo-convex to 

 the superficially double concave, and to the deeply concave instances 

 before us. 



" In the lizards, with the exception of the monitors, we have the 

 same order of phseaomena. 



" We should therefore have a right to conclude that the double con- 

 cave system is more ancient than the concavo-convex, and that the 

 deep concavity indicates an earlier state of existence than the superfi- 

 cially concave ; seeing that the Durdham Down specimens are more 

 ancient than those of the chalk, of Honfleur, Caen, Sussex, Monheim, 

 the Jura, &c. &c. 



" To carry this disquisition further, to attempt to show a succession 

 of creations or changes from the ichthyoid type to the crocodilians 

 (ascending as in the diagram), would require an infinitely more pro- 

 found acquaintance with the subject than is at present attained. 



" We nevertheless think ourselves justified in the assumption that 

 the saurian type approaches the ichthyoid by two parallel lines, the one 

 represented by the crocodiles, the other by the lizards." 



Recent Crocodile. 



Lumbar Vertebra. 



First Caudal V. 



Second Caitdal V. 



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