VERTEBRATE ANIMALS MAMMALIA DIAGRAM 3 29 



have 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 Pachydermata form an order which derives its name 

 from two Greek words meaning u 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 all two hoofs 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 tha^ 

 she is eating afresh the food she 

 has previously swallowed. This 

 Skull of Ox. is rumination. " Digestion is not 



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

 malia ; they have a very complicated stomach, orjrather four 

 stomachs between the end of the oesophagus and the begin, 

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



