594 



THE UNIVERSE. 



The oolite abounds in molluscs, polypoids, and fossil 

 plants. Insects and crustaceans are also found in it. 



The last group of the secondary strata, the cretaceous 

 or chalk formation, plays an important part in geology, 

 partly owing to its depth, partly to the great extent over 



280. Meriaii's OiJOssiim —Didelphis dorsigera (Linnseus). 



which it is found. It is scarcely necessary to say that it 

 owes its name to the chalk (carbonate of lime), of which 

 it almost entirely consists. The cretaceous strata form 

 many of our mountain chains. 



During this period both land and sea appear to have 

 l^een still under the domination of reptiles of colossal size. 

 The Mosasaurus, long called the "great Maestricht animal," 

 an immense marine lizard, attained a length of twenty 

 metres (more than sixty-five feet), whilst contemporary 



