Ornithorhynchus (Gr , a bird, and , a beak), a genus of implacental mammals of the order monotremata, which seem to form a connecting link between mammals and birds, and in some respects having affinities even with reptiles. A single species only is described, the platypus anatinus (Shaw), or ornithorhynchns paradoxus (Blumenb.), the duck-billed platypus of English writers, the water mole of the colonists, and the mallan-gong of the natives; it inhabits the fresh-water streams of Australia and Papua. It is from 18 to 22 in. from the end of the jaws to the point of the tail, the latter being about 5 in.; the color above varies from ruddy to dark brown, and is whitish below; the jaws are enclosed in a horny sheath, very sensitive, like the bill of a duck, and have two horny teeth on each side above and below, fiat, rootless, composed of perpendicular horny tubes; the snout is flat and broad, the lower jaw the narrower and shorter and provided with lamellae on the sides; the eyes small and brilliant; ears not apparent externally, with an aperture which can be opened or shut at will; the tongue consists of two parts, the posterior broad, fiat, with soft papilla) and a free process bearing two pointed horny teeth, the anterior narrow and covered with upright points longest and sharpest toward the tip; the nostrils are at the end of the upper mandible; cheek pouches are present, and a bulb on the back of the tongue prevents the contents of the mouth from passing into the larynx; the fur is soft and thick, like that of the otter.
The legs are short, and the feet five-toed, webbed, and furnished with strong claws; the fore feet are the strongest, and their loose webs extend beyond the claws; the hind legs are armed with a sharp, conical, bony spur, with a corneous investment, perforated for the passage of a duct communicating with a gland situated on the thigh; the tail is flat, broad, and beset with rigid hairs. As the name of the order imports, the alimentary, urinary, and reproductive organs open into a common cloaca, as in birds; mammary glands are present, secreting milk for the nourishment of the young, which are born blind and naked; there are no prominent nipples, and the mammary openings are contained in slits in the integument; the beak in the young is short and flexible, adapted for sucking; M. Verreaux (Revue zoologique, 1848) says the young, when they are able to swim, suck in the milk from the surface of the water into which it is emitted. The shoulder bones are unlike those of other mammals, and are intermediate in arrangement between those of birds and reptiles; in many points of the generative system, also, there are ornithic and reptilian affinities.
It burrows in the banks of streams, where it passes the day in sleep rolled up like a ball, coming out at dusk and during the night in search of food; it is an excellent swimmer and diver, and feeds upon worms, insects, and small aquatic animals, in the manner of a duck; it walks very well, and climbs trees with facility; the burrows, which have an opening below the water, are sometimes 20 or 30 ft. long, extending upward beyond the reach of inundations; in the highest and dry est part is an enlarged cavity for the nest of themselves and young. It can remain under water only about seven or eight minutes at a time; it is cleanly in habit, and fond of warmth and dryness. The young in confinement are playful, and will eat rice and egg, soaked bread, and finely chopped meat; they are rather delicate, and die very soon from want of food. They do not lay eggs, but are true mammals; the fluid secreted by the femoral gland is not poisonous. Skins of this animal are not uncommon, but its skeleton is rare.