360 MAMMALS. 



are distinguished by the following peculiarities : They all breathe 

 air by means of lungs, suspended freely in a cavity, which is sepa- 

 rated from the abdomen by a muscular partition called the dia- 

 phragm, the movements of which, by enlarging the chest, are the 

 principal cause of the inspiration of air. The heart is double 

 that is, consists of two ventricles for the propulsion of blood 

 through the arteries, and two auricles for its reception from the 

 veins. The mouth is closed by fleshy lips, and the skin, with .but 

 few exceptions, is covered with hair. The teeth in Mammals arc 

 organs of great importance to the zoologist. They are generally 

 placed in single series, and vary much in their form, according to 

 the nature of the food, as well as according to their position in 

 the mouth. In man, there are in each half of each jaw two front 

 teeth, having the name of incisors or cutting-teeth ; one, more 

 pointed, called the canine or dog-tooth, or sometimes the eye- 

 tooth ; two somewhat flattened at the top, with single fangs, called 

 false molars ; and three, situated behind all the rest, called true 

 molars or grinders. For simplicity, naturalists have invented the 

 following tabular method of expressing the number and arrange- 

 ment of the teeth, which is called the dental formula : 



meaning incisors, two on each side in the upper and two on each 

 side in the lower jaw ; canines, one on each side in the upper and 

 one on each side in the lower jaw ; false molars, two on each side 

 in both upper and lower jaws ; molars, three on each side in both 

 jaws making in all thirty- two teeth. 



In those races which feed exclusively on flesh, the molar teeth 

 partake of a cutting character, while in those that subsist on grain 

 and herbage the molar or grinding structure prevails. Sometimes 

 the incisors are curiously developed : in the squirrels, rats, and 

 other similar animals, they project forwards, and are continually 

 growing ; in the elephant they stand out in the form of huge 

 curved tusks ; and in the narwhal one is commonly suppressed, 

 while the other grows into a long, spirally-twisted straight tusk, 

 that projects like a horn in front of its head. The whale has no 

 teeth, but a series of horny plates, parallel to each other, depends 

 from the upper jaw, and constitutes the valuable substance called 

 " whalebone." In the ant-eaters, and some others of the Eden- 

 tata, there are no teeth at all, while the armadillo has ninety-six, 

 and some of the dolphins have a hundred and fifty. 



