Ornithorhynchus (O. anatinus), called also the Duck-billed Platypus, or Duck-bill, a lowly aquatic mammal from Tasmania and Australia, the sole species of its genus, which constitutes one of the two families that make up.the Monotremes (q.v.). The adult male is about twenty inches long and the female somewhat less. The strange duck-like bill is formed by the horny sheath which covers the expansion of the premaxillary bones and the mandible. In 1888 Mr. Poulton found teeth in an embryo, and in 1889 Mr. Oldfield Thomas found functional teeth in some young skulls which he examined. When worn away by friction these teeth are not replaced by a second set, but horny structures are developed, those in front being sharp-edged, while those at the back of the mouth function as molars. The thick soft fur is glossy brown in colour. Each limb bears five digits, armed with strong claws, and on the fore limbs the membrane between the digits projects beyond the claws, making these limbs admirable swimming organs. The membrane is turned back on the palm when the animal is walking or burrowing. There is a perforated spur on the hind foot in the young, which disappears in the female. It communicates with a gland, and seems to be a poison organ, but there is only one recorded instance of its use, which is probably limited to certain seasons. The burrows are made in river banks, and have two entrances, one above and one below the water-level. These animals feed on insects, molluscs, and worms, which they take under water and stow away in their cheek pouches, rising from time to time to masticate and swallow their prey. Their sense of smell appears to be fairly acute. The aborigines always asserted that these animals laid eggs, and their story was believed by many Europeans, but no proof of its truth was obtained till 1884, when Mr. W. H. Caldwell's discoveries put the matter beyond doubt, and made it clear that two eggs are laid at a time, each about f inch long, with a "soft" shell, probably broken by the bill of the young.