MAMMALIA. 



289 



bird. It has a mouth resembling the bill of a Duck, and 

 several other peculiarities, which lead to the consideration 



FIG. 60. PLATYPUS, OK DUCK-BILL (Ornithorynchus). 



of it as one of the connecting links between the forms 

 of the Mammalia and Birds. 



3. The RODENTIA comprise the Eat, Beaver, Babbit, Hare, 

 Mouse, &c. They live upon almost any kind of vegetable 

 matter ; the Hat and Mouse will eat anything they can get. 

 Their incisor teeth are two in front of each jaw, and are 

 chisel-shaped and very sharp ; they are so constructed that 

 they shall be kept sharp by being used, which is effected by 

 the hardest part or enamel being in front in a thin layer, so 

 that it is not so easily worn away as the bony part. If these 

 animals be fed on soft food, their teeth continue to grow 

 nevertheless, and protrude from their mouths in a most 

 singular way. The Beaver (fig. 61) is almost aquatic in its 

 habits, it gnaws through the stems of trees, and uses them 

 to dam up the water of the stream they inhabit, so as to 

 secure a constant supply. They build houses or huts of 

 wood, mud, and stones, which they unite in constructing. 

 These houses have their entrances beneath the water, so 



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