442 THE MESOZOIC PERIODS 



of the former group are still living in our modern seas. The Star- 

 fishes also assume their modern condition. Brachiopods are far 

 less abundant and diversified than they had been in the Palaeozoic, 

 and belong, for the most part, to different families, while the Bi- 

 valve and Gastropod Mollusca increase to a wonderful extent. 

 Especially characteristic are the marvellous wealth and variety of 

 the Ammonoid Cephalopods, which disappear at the close of the 

 era. The Dibranchiate Cephalopods, with internal shells, make 

 their first appearance in the Mesozoic, and one group of them, 

 the Belemnites, is almost exclusively confined to the era. The 

 Arthropods show the same revolutionary changes. Among the 

 Crustacea, the Trilobites and Eurypterids have gone out, but all 

 the modern groups are well represented, though many of the 

 Mesozoic genera are no longer to be found in the seas of to-day. 

 Insects reach nearly their modern condition, so far as the large 

 groups are concerned, butterflies, bees, wasps, ants, flies, beetles, 

 etc., being added to the older orthopters and neuropters. 



Fishes become modernized before the close of the era, the 

 Bony Fishes having acquired their present predominance. The 

 Amphibia take a subordinate place, and after flourishing for a 

 time, the great Stegocephala die out, leaving only the pygmy sala- 

 manders and frogs of the present. Birds and Mammals make 

 their first appearance, the former advancing rapidly to nearly their 

 present grade of organization, though not reaching their present 

 diversity, while the mammals remain throughout the era very 

 small, primitive, and inconspicuous. The most significant and 

 characteristic feature of Mesozoic life is the dominance of the 

 Reptiles, which, in size, in numbers, and in diversified adaptation 

 to various conditions of life, attain an extraordinary height of de- 

 velopment. The Mesozoic is called the " Era of Reptiles," because 

 these were the dominant forms of life. They filled all the roles now 

 taken by birds and mammals ; they covered the land with gigantic 

 herbivorous and carnivorous forms, they swarmed in the sea, and, 

 as literal flying dragons, they dominated the air. At the present 

 time there are only four orders of reptiles in existence, and of 

 these only the crocodiles and a few snakes attain really large size. 



