The Duck-billed Platypus is, as every one knows, an aquatic animal. It is not found all over Australia, but is limited to the southern and eastern parts of that continent, and to Tasmania. The animal excavates a burrow for itself in the bank of the slow streams which it frequents. The burrow has one opening below the water and one above; and it is of some length, twenty to fifty feet. The Platypus feeds upon animal food, chiefly "grubs, worms, snails, and, most of all, mussels." These it stows away when captured into its capacious cheek-pouches. The food is then chewed and swallowed above the surface as the animal drifts slowly along. Dr. Semon, from whose work, _In the Australian Bush_, this account of the animal's habits is quoted, thinks that in the nature of the food of the creature the explanation of the loss of the teeth is to be found. He is of opinion that for cracking the hard shells of the mollusc _Corbicula nepeanensis_, upon which _Ornithorhynchus_ mainly feeds, the horny plates are preferable to brittle teeth. _Ornithorhynchus_ is apparently not eaten by the natives by reason of its ancient and fish-like smell. Besides, it is hard to catch on account of its diving capacities, which are aided by an acute sense of sight and of hearing. When the Duck-bill was first brought to this country it was believed to be a deliberate fraud, analogous to the mermaids produced by neatly stitching together the forepart of a monkey and the tail of a salmon. * * * * * {116} CHAPTER VI INTRODUCTION TO THE SUB-CLASS EUTHERIA SUB-CLASS II.--_EUTHERIA_ DEFINITION.--Mammalia with teats. Mammary glands of sebaceous type. Heart with entirely membranous and complete right auriculo-ventricular valve. Brain generally with a corpus callosum. Coracoid much reduced and not reaching sternum. No interclavicle. Vertebrae with epiphyses. Ribs double-headed. Viviparous, with a small ovum. In this group are included not only the Eutheria in the sense of Huxley, but also his Metatheria. Though the Metatheria, or Marsupials as we shall term them, undoubtedly form a most distinct order of mammals, perhaps even a trifle more distinct than most others, their differences from the remaining tribes are not by any means so great as those which separate _Ornithorhynchus_ and _Echidna_ from all other mammals. In his well-known memoir upon the arrangement of the Mammalia,[64] Professor Huxley enumerated eleven characters as distinguishing the Metatheria either from the Prototheria or from the Eutheria. Of these only three were characters in which they approach the lower mammals. According to his showing, therefore, the preponderance of marsupial features are Eutherian. The three characters of Prototherian type are (1) the presence of epipubes; (2) the small corpus callosum; (3) the absence of an allantoic placenta.