60 THE ORIGIN OF MAN 



greatly modified. The bivalves and gastropods 

 were also undergoing a great change and the am- 

 monids of the Permian gave rise to a wonderful 

 evolution but were almost exterminated at the 

 close of this period. The lung-fishes of the Trias- 

 sic were greatly reduced in number and the dom- 

 inant forms were the ganoids with the bony fishes 

 advancing toward our present type. 



The vertebrates of the land were now very 

 varied exhibiting much structural and adaptive 

 progression over their late Paleozoic ancestors. 

 Crocodile-like reptiles of the sprawling type and 

 other active forms were common, but the dino- 

 saurs, however, were the lords of the land and 

 they were present in great variety and in great 

 size. Some were herbivorous; others were car- 

 nivorous; some walked on all fours while others 

 walked occasionally or habitually on two legs, 

 after the manner of birds. We have seen how 

 freshwater vertebrates were forced to adapt 

 themselves to the land and now that the reptiles 



