332 GENERAL ZOOLOGY 



seas, became extinct and the ancestors of decapod crusta- 

 ceans appeared. The land vegetation consisted largely of 

 coniferous trees. 



At the beginning of the Palaeozoic Age, animals were rep- 

 resented only by invertebrates, but toward its close fish- 

 like chor dates, amphibians and reptiles arose, and gained 

 prominent places. At the beginning also, most of the 

 animals were marine, but, though the ocean continued to 

 be abundantly populated, as life progressed in specialization 

 there was an increasing invasion of fresh-water and land 

 habitats. Many groups that were important in Cambrian 

 times had dwindled (Brachiopoda) or become extinct (Tri- 

 lobita) at the close of the Permian. 



MESOZOIC AGE 



Triassic Period. During Triassic times the land was 

 covered by great forests of coniferous trees, mixed with the 

 older club mosses, ferns, and algae. This luxuriant growth 

 supported numerous insects and such modern types as the 

 beetles had made their appearance. Some of the stego- 

 cephalians at this time were of great size, one species hav- 

 ing a skull measuring more than a yard in length. Reptiles 

 were more diverse and attained greater size than during the 

 preceding period. The giant dinosuars had appeared in 

 various places; heavy-bodied plesiosaurs were beginning 

 to frequent the shores of oceans; whale-like ichthiosaurs 

 competed with fishes in marine habitats; chelonians fre- 

 quented the swamps and rivers. Small mammals also ap- 

 peared in the Triassic period, but did not take an important 

 place among animals until later. 



Jurassic Period. Reptiles rose to the zenith of their 

 development in this period and were distributed in all parts 

 of the earth. Herbivorous and carnivorous dinosaurs 

 (Fig. 101) dominated the land and frequented the borders 

 of marshes; tortoises were numerous along rivers; the ocean 



