76 ’ The Sea 
Proterozoic Era 
(began about 700 million years ago) 
This era is almost devoid of life except for various low 
forms; various kinds of algae, etc. Extreme north and south 
regions of the earth were apparently in glacial climates. 
Lower Paleozoic Era 
(began about 450 million years ago) 
During this era the first marine faunas appear, the most 
abundant being trilobites, fossils of which are found in vast 
quantities. This is the time when hard shelled animals 
thrived. As the era progressed armored fishes, corals, nauti- 
loids appeared. 
Middle Paleozoic Era 
(began about 315 million years ago) 
The first terrestrial air breathers (scorpions, lung fishes ) 
made their appearance, and also the first land plants, and 
fresh water fishes. 
Upper Paleozoic Era 
(began about 240 million years ago) 
The formation of coal beds from rich jungle foliage, the 
death and extinction of earlier animal species; rise of in- 
sects and spiders, primitive snakes and other reptiles, and the 
ancient sharks, whose teeth we still dredge from the ocean 
bottom. 
Mesozoic Era 
(began about 150 million years ago) 
This is the era of flowering plants, of birds and winged rep-— 
