312 PRACTICAL LESSONS IN SCIENCE. 



The Eodentia are a numerous group of small animals charac- 

 terized by their chisel-shaped incisor teeth and the broad com- 

 pact skull furnishing attachment and leverage for the powerful 

 muscles used in their work of gnawing. The dental formula is 

 two incisors and six or eight grinders in each jaw. The hares and 

 rabbits having four incisors in the upper jaw, and some Aus- 

 tralian forms have only four grinders. The incisors are enameled 

 on the anterior surface with dentine on the posterior, and as the 

 dentine wears away faster the tooth is always sharp and chisel- 

 shaped. The teeth grow continuously during life about as fast 

 as they wear away, so that they are always the same length. 

 When one incisor is destroyed in any way its opposite unopposed 

 continuing to grow has been known to form a complete ring, 

 sometimes causing the death of the animal by preventing the use 

 of its other teeth. These animals are compactly built, very pro- 

 lific, and often showing great intelligence in the building of homes 

 and the storing of food. Some migrate at the beginning of win- 

 ter; some take a winter sleep. They are vegetarians, feeding on 

 roots, seeds, fruits, etc. The mammals already studied walked 

 on their toes, were in general digitigrada, but the rodentia walk 

 on the palm of the hand or sole of the foot, and are plan tigrada. 

 The digits are freely movable, having claws or nails. In this 

 order belong the hares and rabbits with long ears, powerful hind 

 legs and short tail; the guinea pig and agouti, the porcupine 

 with blunt nose and sharp quills ; the chinchillas and the jumping 

 mice; the true rats and mice and their allies; the voles with their 

 short hairy ears and tail, and the lemming, the beavers, dormice 

 and squirrels. 



The Insectivora are small plantigrade animals whose feet are 

 furnished with strong claws ; the elongated head often ends in a 

 pointed snout and the jaws carry a full set of teeth; the den- 

 tition varies considerably, the most distinctive character being 

 the tubercles or points on the crowns of the grinders and that 

 the canines are not prominent. They feed on small animals, 

 insects and worms. 



The Erinacidae or Hedgehogs with stiff bristles and spines. 



