Fam. { 76. Didelphidæ Opossums. { 77. Dasyuridæ Native Cats. Rapacia (Wagner) { 78. Myrmecobiidæ Native Ant-eater. { 79. Peramelidæ Bandicoots. Poephaga (Owen) 80. Macropodidæ Kangaroos. Carpophaga (Owen) 81. Phalangistidæ Phalangers. Rhizophaga (Owen) 82. Phascolomyidæ Wombats. Order--MONOTREMATA. The last order, the Monotremata, consist of two families, which Professor Carus combines into one, but which it seems more natural to keep separate. Fam. 83. Ornithorhynchidæ Duckbill. 84. Echidnidæ Echidna. {92}BIRDS. Birds are perhaps the most difficult to classify of all the divisions of the vertebrata. The species and genera are exceedingly numerous, and there is such a great uniformity in general structure and even in the details of external form, that it is exceedingly difficult to find characters by which orders and families can be characterised. For a long time the system of Vigors and Swainson was followed; but this wholly ignored anatomical characters and in many cases plainly violated well-marked affinities. Characters derived from the form of the sternum, the scutellation of the tarsi, and the arrangement of the feathers, have all assisted in determining natural groups. More recently Professor Huxley has applied the variations of the bony palate to the general arrangement of birds; and still more recently Professor Garrod has studied certain leg-muscles for the same purpose. The condition of the young as regards plumage, and even the form, texture, and coloration of the egg, have also been applied to solve doubtful cases of affinity; yet the problem is not settled, and it will probably remain for another generation of ornithologists to determine with any approach to accuracy what are the most natural divisions of the class into orders and families. In a work like the present it is evidently not advisable to adopt all the recent classifications; since experience has shown that no arrangement in which one set of characters is mainly relied on, long holds its ground. Such modifications of the old system as seem to be well established will be adopted; but the older groups will be adhered to in cases where the most recent classifications are open to doubt, or seem inconvenient as separating families, which, owing to their similarity in general structure, form and habits are best kept together for the purposes of geographical distribution.