624 HISTORICAL GEOLOGY 



In general, it can be said that the earliest Tertiary birds are typi- 

 cal modern birds, modified for various conditions of life, some being 

 aquatic, some waders, and some land birds; and that the changes 

 which took place during the period resulted in the production of 

 modern genera and species. Although 12,000 species of birds are 

 living to-day, less than 500 are known from the Tertiary. The 

 reason for the rareness of bird remains, as has already been discussed 

 (p. 563), is probably due to their lightness, which causes them to 

 float and thus exposes their carcasses, often for many days, to fish 

 and other carnivorous animals. 



REFERENCES FOR BIRDS 



Hutchinson, H. N., — Extinct Monsters and Creatures of Other Days, pp. 220-224. 



Lucas, F. A., — Animals of the Past, pp. 138-158. 



Osborn, H. F., — Age of Mammals, pp. 151, 152, 195, 257, 450. 



Woodward, A. S., — Vertebrate Paleontology, p. 244. 



Zittel-Eastman, — Textbook of Paleontology, Vol. 2, pp. 268-278. 



Reptiles and Amphibians. — The usual practice at present is to 

 include in the North American Tertiary no formations containing 

 dinosaur remains. This is the custom even though a great uncon- 

 formity exists in the Laramie (p. 517) of the western interior of 

 North America, which, were it not for the presence of dinosaur fossils 

 in the formation (Lance) overlying the unconformity, would doubt- 

 less be considered the dividing line between the Mesozoic and Ter- 

 tiary. The most conspicuous reptilian survivors of the Mesozoic 

 were the turtles, crocodiles, and large river lizards (Champsosaurus), 

 the last, however, disappearing early in the period. Snakes began 

 in the Cretaceous, having doubtless been derived in the Mesozoic 

 from lizards, by the degeneration of their limbs. A number of species 

 have been found in the Tertiary, few of which, however, are to be 

 distinguished from those now living, and the majority belong to the 

 non-poisonous varieties. Some of the early sea snakes attained a 

 length of about 20 feet. 



Among amphibians, salamanders, newts, frogs, and toads occur in 

 Oligocene deposits; and numerous impressions of tadpoles have 

 been preserved. 



Deductions as to the climate of the Tertiary can be made from the 

 reptilian life as well as from the vegetation (p. 634). Crocodiles, 

 lar^e and small, and of several genera, lived in abundance in the 

 Middle Eocene (Bridger) of the western interior and suggest a 



