THE MONOTEEMES 



237 



jaws of which show the same modifications consequent upon 

 entire loss of teeth as do those of the Anteaters already dis- 

 cussed, avc turn our attention to the transient dentition of 

 Ornithorhynchus. 



This fossorial semi-aquatic animal possesses peculiar jaws 

 armed in the adult not with teeth but with horny thickenings 

 of the epithelium. For crushing the shells of fresh-water mol- 

 luscs upon which the animal feeds teeth would be poor and 

 brittle instruments. On the other hand the nutcracker-like 



Fig. 84. — Dentition of Duck-billed Platypus (Ornithorhynchus anatinus. enlarged, 

 after Oldfield Thomas). 1. Left upper tee'th. 2. Left lower teeth. The third tooth 

 is not represented; the first is very aberrant but in the second may be traced indications 

 of an originally tritubercular tuberculo-sectorial type. 



jaws with their horny covering are excellently adapted for 

 grinding .such food. Notwithstanding the fact that there are 

 no teeth in the adult, actual teeth corresponding to the perma- 

 nent dentition of other Mammals and even vestiges of a decidu- 

 ous dentition have been discovered. The "•permanent" teeth are 

 three on each side of each jaw, twelve in all and after great 

 attrition from which they are worn to the thickness of paper 

 they are shed when the animal is about two-thirds grown. 

 The two anterior teeth are large but the third very small 



