MAMMALIA 



315 



258) vary in size, shape, and number, depending on the en- 

 vironmental habits. In all but the Cetacea the ends of the digits 

 are protected by horny epidermal coverings, variously termed 

 hoofs, nails, or claws. Rudiments of these are found in the 

 embryos of the Cetacea, thus suggesting their terrestrial origin. 

 The Digestive Organs. One of the peculiarities of the 

 digestive organs in mammals is the development of the salivary 

 glands. The teeth are "heterodont" (Fig. 254) instead of 

 " homodont," as in some of the lower classes of chordates. 



Fig. 258. Posterior limb of man, monkey, dog, sheep, and horse: 1, Hip- 

 joint; 2, thighbone; 3, knee-joint; 4, bones of leg; 5, ankle-joint; 6, bones 

 of foot; 7, bones of toes. (Le Conte's "Geology," American Book Co., 

 Publishers.) 



Some mammals have no teeth, at least not in the adult stage. 

 The teeth are attached to the premaxillary, maxillary, and den- 

 tary bones, and to no others. These heterodont teeth may be 

 classed as incisors, canines, premolars, and molars. The teeth 

 would indicate an omnivorous feeding habit, but this is not true 

 for all mammals. Some are omnivorous, some carnivorous, and 

 many herbivorous. The teeth often give a hint as to the feed- 

 ing habits, as well as proving an important element in classifica- 

 tion. 



