VERTEBRATE ANIMALS MAMMALIA DIAGRAM o. 29 



liave acquired their full size, the incisors of rodents keep on 

 growing all their life, as fast as they are worn away. This may 

 be verified by cutting the teeth of a rat or a rabbit, when they 

 will very soon regain their length. 



We may mention among the rodents, the squirrels, the dor- 

 mice, the moles, the marmot, the family of the rats, the field- 

 mice, the beaver, the porcupine, and the family of the hares. 



5th. Next come the Edentata. These are animals which 

 inhabit tropical countries. They have no incisor teeth nor 

 canines ; and some of them have no teeth at all. They are 

 seldom brought to Europe. 



6th. The Packydermata form an order which derives its name 

 from two Greek words, meaning "thick skin." Nearly all are 

 large animals with a thick skin, and never having the feet 

 simply cloven like the ruminants. The elephant, the rhinoceros, 

 the horse family, the wild boar, and the hog are placed in the 

 order of pachyderms. 



7th. The order Ruminantia com- 

 prises a great many animals which 

 have a cloven hoof to each foot. 

 Many have incisors only in the 

 lower jaw, and none in the upper; 

 and alone of all the mammalia, 

 they ruminate. We often see a cow 

 lying down in the fields motionless, 

 and masticating all the time, al- 

 though she crops no grass. On 

 opening her mouth, we see that 

 she is eating afresh the food she 

 has previously swallowed. This 

 is rumination. Digestion is not Skull of Ox. 



effected in ruminants in the same way as in other mam- 

 malia ; they have a very complicated stomach, or rather four 

 stomachs, between the end of the oesophagus and the begin- 

 ning of the intestines. The first and largest is called the 



