120 COMPAEATIVE DENTAL ANATOMY 



horny structures which take the place of teeth. 

 The Ornithorynchus is a mammal living in Aus- 

 tralia with a bill like a duck, and like the duck's 

 bill transversely ridged to strain food from the 

 water. Each jaw is provided with four horny 

 teeth, of which the two front ones have long 

 ridges and the back ones are tubercular and cal- 

 careous, somewhat like true grinding teeth. 

 Their structure is a fibrous, horny substance, like 

 hoofs or nails, with only three per cent, of earthy 

 matter. The other member of the order, the 



FlG. 38. — -Duck-billed Platypus (Ornithorynchus anatinus) . 



Echidna, has an edentulous bill, but has the 

 tongue and roof of the mouth covered with horny 

 spines to crush the ants caught by its viscous 

 tongue. 



The next highest order of mammals is that of 

 the Bruta, which comprises the Sloths, Armadil- 

 los, Ant-eaters, etc., which are of very low or- 

 ganization. They were first called Edentata, — 

 without teeth, — and later were described as be- 

 ing without incisor teeth, as median incisors are 

 absent in all the members of the order. The 

 teeth are also destitute of enamel, the dentin alone 



