Order 3 Teleosts Bony fishes; have scales in skin; air bladder. 

 Examples, salmon, herring, perch, cod, flounder (see illustrations, 

 pp. 173, 210 and 421). 



Order 4 Dipnoi ("double breathers"). Fishes with lunglike struc- 

 tures, as well as gills; certain species skip over mud flats when tide is 

 out;' others burrow, in mud and live through the hot dry season in a 

 mucus-lined cocoon. Found only in the Southern Hemisphere. 



CLASS 2 AMPHIBIANS ("double life"). Breathe by means of gills in 

 early stages, familiar to us as tadpoles, and later develop lungs; have bony 

 skeleton with two pairs of appendages and a three-chambered heart; cold- 

 blooded; skin is without scales. Examples, frog, toad, newt, salamander, mud 

 puppy (see illustrations, pp. 211, 309, 355, 379 and 421). 



CLASS 3 REPTILES ("crawl"). Wholly air-breathers; dry scaly skin; 

 four-chambered heart; cold-blooded; eggs large, with a membranous cover- 

 ing. Four orders are usually recognized : 



Order 1 Chelonia Protective shell composed of bony plates covered 

 with horny plates; toothless jaws. Examples, turtles and tortoises. 



Order 2 Serpents Reptiles without legs. Examples, snakes, adders, 

 cobras (see illustrations, pp. 4 and 422). 



Order 3 Lacertilia Body and tail usually long and slender, with 

 four legs. Examples, lizards, chameleons, horned toad, Gila monster, 

 glass snake (see illustration, p. 230). 



Order 4 Crocodilia Large, semiaquatic, four-legged animals; though 

 air-breathers, can remain under water for five or six hours without 

 drowning. Examples, alligators, crocodiles, caymans, gavials. 



CLASS 4 AVES ("birds"). Warm-blooded; four-chambered heart; cover- 

 ing of feathers; front limbs are wings; air spaces in bones; no diaphragm; 

 eggs have limy shells; horny beak, no teeth. 



Living species of birds can be conveniently divided into the running, or 

 flightless, birds (ostrich, cassowary, emu) and the Jlying birds. In this classi- 

 fication the more important orders of flying birds have been grouped, so far 

 as possible, according to their habitats, since the shapes of the limbs and beak 

 are so distinctly associated with the mode of life. Some of the more im- 

 portant orders of the flying birds are listed below, with examples of typical 

 families (see illustrations, pp. 30, 178, 293, 362, 392 and 649). 



Order 1 Divers. Loon family, grebe family. 



Order 2 Tube-nosed swimmers. Shearwater and petrel family, storm- 

 petrel family. 



Order 3 Pelican tribe. Tropic- bird family, pelican family, gannet and 

 booby family, cormorant family. 



Order 4 Storklike birds. Heron and bittern family, stork and wood- 

 ibis family, ibis and spoonbill family, flamingo family. 



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