PALEONTOLOGY 



297 



ERAS OR 

 PERIODS. 



AGES OR SYSTEMS. 



ANIMALS ESPECIALLY CHARACTERISTIC 

 OF THE ERA OR AGE. 



Cenozoic. 



Era of 

 Mammals. 



Quaternary or Pleis- 

 tocene (age of man 

 and insects) 



Tertiary: Pliocene, 

 Miocene, Eocene. . . . 



Man; mammals, mostly of spe- 

 cies still living. 



Mammals abundant; belonging to 

 numerous extinct families and 

 orders. 



Cretaceous. 



Mesozoic. 



Era of 

 Reptiles. 



Jurassic 

 Triassic 



Birdlike reptiles; flying reptiles; 

 toothed birds; first snakes; bony 

 fishes abound; sharks again 

 numerous. 



First birds; giant reptiles; ammo- 

 nites; clams and snails abun- 

 dant. 



First mammals (a marsupial); 

 sharks reduced to few forms; 

 bony fishes appear. 



Paleozoic. 



Era of 

 Invertebrates. 



Carboniferous (age of 

 amphibians) 



Devonian (age of 

 fishes) 



Silurian (age of inver- 

 tebrates) 



Ordovician or Lower 

 Silurian 



Cambrian. 



Earliest of true reptiles. Am- 

 phibians; lung fishes; fringe 

 fins; first crayfishes; insects 

 abundant; spiders; fresh-wa- 

 ter mussels. 

 First amphibian (froglike ani- 



I mals); sharks; ostracophores; 

 first land shells (snails); mol- 



[ lusks abundant; first crabs. 



f First truly terrestrial or air- 

 breathing animals; first in- 

 sects; corals abundant; mailed 

 fishes. 



{ First known fishes, ostraco- 

 phores, mailed and with carti- 

 laginous skeleton; brachio- 

 pods; trilobites, mollusks, etc. 



Invertebrates only. 



Archean. 



Algonkian. Lauren- j $ [mple mar ine invertebrates, 

 tian 



visions of geological time some one class of animals was espe- 

 cially numerous in species, and was evidently the dominant 

 group of animals through that period. The different ages are 

 therefore spoken of in terms of the prevailing life. Thus, the 

 "Silurian Age" is known as the age or era of invertebrates; 

 the " Devonian/' as the age of fishes. In the same way we 

 have the "Reptilian Age/' the "Mammalian Age/ 7 according 



