310 MONOTREMATA. 



narrow at the angle of the mouth, but immediately 

 below the angle, its width is about three quarters of 

 an inch. This membrane folds back over the fore 

 part of the head and throat, and probably serves to 

 keep the mud, — in which these animals are (like 

 ducks) constantly grubbing in search of food, — from 

 working into the fur ; it may also serve somewhat to 

 protect the eyes. The nostrils are round and rather 

 large, placed near to each other and situated on the 

 upper surface near the extremity of the beak, — that 

 is, they are just within the boundary of the bony 

 portion. The lower mandible has the same general 

 form as the upper, but is of a much smaller size ; it 

 is nearly flat, being but slightly convex externally, 

 and concave internally; like the upper mandible, it is 

 surrounded by free membrane, but this is compa- 

 ratively narrow; that at the sides presents numerous 

 transverse ridges, which serve, no doubt, a similar 

 purpose as those which we observe in the same situ- 

 ation in the under mandible of the ducks. The only 

 teeth vv'hich this animal possesses are four molars, 

 two on each side of each jaw ; these teeth, unlike 

 those of other quadrupeds, consist of a horn-like 

 substance, are situated far back in the head and 

 present a broad and uneven masticating, or rather 

 crushing surface, and are of an irregular subquadrate 

 form. The eyes are very small, of a light brown colour, 

 and situated about half an inch behind the base of 

 the beak. " The external orifice of the ears is situ- 

 ate near the upper part of the external angle of the 

 eye. When a living specimen is examined the orifice 



