EGG-LAYING MAMMALS OR MONOTREMES. 161 



with a thick coat of fur, generally of a full umber- 

 brown colour, and inclining either to rufous or to black 

 in different individuals ; the under-parts and a ring 

 round the eye being yellowish, or grayish- white. The 

 tail is broad and flat, and thus well adapted to act as a 

 rudder in swimming ; while the feet are fully webbed, 

 and thus proclaim the aquatic habits of the creature. 

 The beak is black above, and yellow and black below. 

 The limbs are remarkable for their shortness, their 

 length being even insufficient to keep the body from 

 touching the ground. 



In the adult animal there are no teeth in the jaws, 

 their function being performed by a few hard and flat 

 horny plates, aided by the horny edges of the jaws 

 themselves. In young individuals there are, however, 

 a small number of well-developed teeth in the hinder 

 half of each jaw, which fall out before the animal 

 attains its full dimensions. The structure of these 

 teeth is unlike that found in any other living mammal ; 

 the chief peculiarity being the presence of a groove run- 

 ning from back to front, and bordered by ridges bearing 

 cusps on either side. The presence of these deciduous 

 teeth has only recently been made known ; and we 

 may safely infer that the ancestors of the Duck-bill 

 were provided with a full set of permanent teeth, which 

 have gradually tended to disappear. This rudimentary 

 condition of the teeth in the Duck-bill is indeed another 

 instance of that tendency to the loss of the teeth in 

 certain groups of animals to which allusion has been 

 made in Chapter VII. ; and the complete loss of the 



