READING 

 REFERENCES 



restricted fields. An instance of this is seen today in the 

 four groups of Mesozoic reptiles. Into these four groups, 

 by the means of careful and systematic classification, over 

 40,000 described species have been placed. 



The following division of the vertebrates into dis- 

 tinct classes has become established. 



Class I. The AGNATHEA: The Crustacean- ^STand 

 Ostracoderm Quasi- Vertebrates. (Chamberlin and Salis- 

 bury, Vol. II., pp. 482-486). All fossil forms. 



Class II. The PISCES: The Fishes. (Pirsson and 

 Schuchert, Vol. II., pp. 678-694). Living and fossil forms. 



Class III. The AMPHIBIA: The Frogs, Toads, 

 Salamanders, Stegocephalia, etc. About 900 living species 

 and 575 extinct fossil species have been described. (Pirsson 

 and Schuchert, Vol. II., pp. 797-804). 



Class IV. The REPTILIA: The Lizards, Turtles, 

 Crocodiles, Snakes, Dinosaurs, etc. There are about 4,500 

 living species, and about 500 extinct species described. 

 (Pirsson and Schuchert, Vol. II., pp. 804-810, 827-828, 

 831-843.) Additional references: Water Reptiles Past and 

 Present, Williston; American Permian Vertebrates, Willis- 

 ton; Permo-Carboniferous Vertebrates From New Mexico, 

 Williston, Case and Mehl; Dinosaurs, Matthew; and East- 

 man's translation of Zittel's Die Grundzuge der Palaon- 

 tologie, Vol. II., pp. 140-255. 



Class V. AVES. The Birds. There are about 

 12,000 living species and about 500 extinct species. (Pirsson 

 and Schuchert, Vol. II., pp. 854-856.) Additional refer- 

 ences : Eastman's translation of Zittel's Die Grundzuge der 

 Palaontologie, Vol. 11, pp. 256-278; Creatures of Other 

 Days, Hutchinson, pp. 150-173; Extinct Monsters, Hutch- 

 inson, pp. 213-224. 



Class VI. MAMMALIA: The Dogs, Sheep, 

 Horses, Men, etc. There are about 3,000 living species and 

 about 1,000 extinct species described. References: The 

 Age of Mammals, Osborn; A History of Land Mammals 

 of the Western Hemisphere, Scott; Extinct Animals, Lan- 

 caster, pp. 103-189; Exinct Monsters, Hutchinson, pp. 135- 

 212, 225-234. Popular, Creatures of Other Days, Hutchin- 

 son, pp. 174-253. Evolution of Mammalian Molar Teeth, 

 Osborn: Tertiary Vertebrata, E. D. Cope, U. S. G. S. Re- 

 port 1884, Hayden Survey. 



