During the Triassic and Jurassic periods the sub-class of the Monotremes was represented by a number of different stem-mammals. Numerous fossil remains of them have lately been discovered in the Mesozoic strata of Europe, Africa, and America. To-day there are only two surviving specimens of the group, which we place together in the family of the duck-bills, Ornithostoma. They are confined to Australia and the neighbouring island of Van Diemen's Land (or Tasmania); they become scarcer every year, and will soon, like their blood-relatives, be counted among the extinct animals. One form lives in the rivers, and builds subterraneous dwellings on the banks; this is the Ornithorhyncus paradoxus, with webbed feet, a thick soft fur, and broad flat jaws, which look very much like the bill of a duck (Figures 2.269 and 2.270). The other form, the land duck-bill, or spiny ant-eater (Echidna hystrix), is very much like the anteaters in its habits and the peculiar construction of its thin snout and very long tongue; it is covered with needles, and can roll itself up like a hedgehog. A cognate form (Parechidna Bruyni) has lately been found in New Guinea. These modern Ornithostoma are the scattered survivors of the vast Mesozoic group of Monotremes; hence they have the same interest in connection with the stem history of the Mammals as the living stem-reptiles (Hatteria) for that of the reptiles, and the isolated Acrania (Amphioxus) for the phylogeny of the Vertebrate stem. The Australian duck-bills are distinguished externally by a toothless bird-like beak or snout. This absence of real bony teeth is a late result of adaptation, as in the toothless Placentals (Edentata, armadillos and ant-eaters). The extinct Monotremes, to which the Promammalia belonged, must have had developed teeth, inherited from the reptiles. Lately small rudiments of real molars have been discovered in the young of the Ornithorhyncus, which has horny plates in the jaws instead of real teeth. (FIGURE 2.269. The Ornithorhyncus or Duck-mole. (Ornithorhyncus paradoxus). FIGURE 2.270. Skeleton of the Ornithorhyncus.)