76 THE ORIGIN OF MAN 



torial waters there arose remarkably aberrant 

 stocks of bivalves, in which one valve was ce- 

 mented to some object, the shell growing upward 

 into a short or long, twisted, thick cone, while the 

 covering valve was either twisted or a thickened 

 and simple hood. These were the chamids and 

 rudistids, which also continued into the Creta- 

 ceous and then gave rise to the caprinids and 

 larger rudistids, shells that were veritable reef- 

 builders. 



The life of the Cretaceous was the culmination 

 of the dinosaurs, pterosaurs and reptilian mam- 

 mals that began in the Triassic, the toothed birds 

 of Jurassic origin and the marine faunas of 

 Jurassic time and it was the final expression of 

 medieval life and out of it arose the modern 

 world of organisms. The invertebrate animals 

 of the Cretaceous were much like those of the 

 Comanchian with the difference that the am- 

 monids were making their last stand as I have 

 explained by the uncoiling and unnatural twist- 



