THE ORIGIN OF MAN 77 



ing of their shells. The belemnids were still 

 present but not so abundant as in the Jurassic. 

 The unionids or pearl shells were common, also 

 freshwater snails and land snails. 



Of this period, the most common and charac- 

 teristic of the herbivorous dinosaurs were the 

 Ceratopsia, which were horned animals, some- 

 times twice as heavy as and larger than elephants. 

 Their name is derived from the Greek keras, sl 

 horn, because they have on their heads two or 

 three large horns. Of all the large theropod dino- 

 saurs there were several kinds, but the most ter- 

 rible of all in size and weight and ferocity was the 

 form known as the King Tyrant Saurian (Ty- 

 rannosaurus rex) probably the mightiest flesh 

 eater of all time, carrying his head sixteen feet 

 above the ground and measuring forty feet along 

 the vertebral column. Flying reptiles or ptero- 

 saurs of early Cretaceous time are known in gi- 

 gantic species. They were without teeth and tail, 

 the head being more than three feet long, and 



