And it is not a little significant that the egg-layers are confined to Australia and New Guinea, the home of many other primitive and ancient types which have disappeared from the rest of the world. In one sense indeed the platypus and the echidna are not exactly primitive creatures, as they have several specialised characters which were evidently wanting in their ancestors. They may rather be described as specialised branches of an ancient and primitive stock. The duckbill is a heavily made aquatic mammal of about the size of a very short-legged rabbit, with blackish, mole-like fur above, passing gradually into whitish beneath, and a short, thick, tapering tail. The very short limbs terminate in thick toes, connected together by a web and armed with strong, pointed claws. In the fore-feet the margin of the web projects considerably beyond the claws, but on the rare occasions that the animal leaves the water the margin is folded downward beneath the claws, so as to leave the claws exposed. In museum-specimens, however, the web is almost invariably shown fully expanded, as in the accompanying illustration; a condition in which it would obviously be impossible for the animal to walk on land. [Illustration] The most remarkable external feature of the duckbill is undoubtedly the duck-like, naked beak, pierced with two holes representing the nostrils. In stuffed specimens, at any rate, this beak is dark-coloured and horny in consistence, and looks as though it did not belong to the animal, but in life it is soft and tender. Medium-sized, dark eyes complete the physiognomy of this strange creature, in which external ears are wanting. The internal arrangements of the mouth of the duckbill are scarcely less curious than the exterior. In early life the mouth is furnished, both above and below, with three pairs of somewhat quadrangular cheek-teeth, with raised and cusped margins. Beneath these grow up certain large horny plates, and about the time that full maturity is attained the teeth become worn out, and are finally shed, thus leaving the horny plates as the sole masticating organs. This replacement of the teeth by horny plates appears to be connected with the nature of the food, for while in early life the duckbill appears to subsist mainly on water insects and other comparatively soft aquatic creatures, later on it takes to feeding almost entirely on bivalve shells of one particular species; and for crushing the stout shells of these molluscs it has been suggested that the tough horny plates are better suited than brittle teeth.