Class Mammalia 571 



unlimited range of possible modes of living. For each mode a specific 

 anatomic complex must be achieved. A survey of the present world's 

 mammals shows that they have gone far in realizing these possibilities. 

 If their thousands of species (whether 6,000 or 18,000 is a matter of 

 opinion) are passed before us in review, a mere casual inspection, with- 

 out detailed anatomic consideration, suffices for recognition of a 

 relatively small number of well-marked types which may be briefly 

 distinguished as follows: 



Two mammals of the Australian region, the duckbill (Orni- 

 thorhynchus: Fig. 532) and spiny anteater (Echidna: Fig. 533), 

 unique in being egg-laying: also in having only one posterior opening, 

 the cloacal aperture — hence called Monotremes. 



Numerous mammals found only in the Australian region (e.g., 

 kangaroos) and a few American mammals (opossums) whose young 

 are born at an early fetal stage and then reared in an external pouch 

 (marsupium) formed by the skin of the mother's abdominal wall 

 (Figs. 534, 539) : Marsupials. 



Small, relatively unspecialized, mostly insectivorous; e. g., the 

 burrowing moles and the hedgehogs: Insectivores. 



Mostly small and herbivorous; strong, chisel-like gnawing incisors: 

 rat, mouse, squirrel, beaver, porcupine: Rodents. 



Flying mammals; mostly insectivorous, some frugivorous : . . . .Bats. 



Teeth lacking or poorly developed; external armor of bony or horny 

 scales in some: armadillo, sloth, South American anteaters, 

 pangolin (Figs. 552-555) : Edentates. 



Large herbivores, most of them having only one or two functional 

 digits on each foot, each digit bearing a horny hoof; horse, pig, ox, 

 sheep, deer, elephant, coney (Hyrax) : Ungulates. 



Large aquatic herbivores; Atlantic and Pacific sea cows (Fig. 

 577) : Sirenians. 



Large, aquatic; fishlike in external form: whale, porpoise, dol- 

 phin : Cetaceans. 



Teeth specialized for eating flesh; dog, wolf, cat, tiger, bear, 

 seal, walrus: Carnivores. 



Mostly arboreal; no extreme anatomic specialization: monkeys, 

 more or less monkey-like lemurs, chimpanzees and other large 

 apes — and, regarded anatomically and impartially, the human 

 mammal must be included here among Primates. 



More extended and precise definitions of the various types of 

 mammals can be given after the anatomic peculiarities of mammals 

 have been discussed. 



