The first and lowest order of the Mammalia is that of the Monotremata, constituting by itself the division Ornithodelphia, and containing only two genera, both belonging to Australia - namely, the Duck-mole (Ornitho-rhynchns) and the Porcupine Ant-eater (Echidna).

The order is distinguished by the following characters: - The intestine opens into a "cloaca" which receives also the products of the urinary and generative organs, which discharge themselves into a urogenital canal - the condition of parts being very much the same as in Birds. The jaws are either wholly destitute of teeth (Echidna) or are furnished with four horny plates which act as teeth (Ornithorhynchus). The pectoral arch has some highly bird-like characters, the most important of these being the extension of the coracoid bones to the anterior end of the sternum. An interclavicle is also present. The females possess no marsupial pouch, but the pelvis is furnished with the so-called "marsupial bones" being special ossifications of the internal tendons of the external oblique muscles of the abdomen. The testes of the male are abdominal throughout life, and there is therefore no scrotum, whilst the vasa deferentia open into the cloaca. The corpus callosum is very small, and has been asserted to be altogether wanting. There are no external ears. The mammary glands have no nipples, and their ducts open either into a kind of integumentary pouch (Echidna) or simply on aflat surface (Ornithorhynchus). The young are said to be destitute of a placenta, or, in other words, no vascular connection is established between the foetus and the mother. The feet have five toes each, armed with claws, and the males carry perforated spurs on the back of the tarsus (attached to a supplementary tarsal bone).

The order Monotremata includes only the two genera Ornithorhynchus and Echidna (or Tachyglossus) - the one represented by a single species (O. paradoxus or anatinus), and the other by four species (E. hystrix, E. setosa, E. Lawesi, and E. Bruijnii). All are exclusively confined to the Australian province.

Fig. 357.   Osteology of Monotremes. A, Skull of Echidna hystrix. B, Side view of the skull of Ornithorhynchus anatinus, and C, lower jaw of the same, viewed from above, showing the horny dental plates (m). D, Sternum and adjacent parts of the skeleton of a young Ornithorhynchus: c c Clavicles; i Interclavicle; p Presternum ; ms, Mesostemum; r r Vertebral ribs; ir Intermediate ribs ; sr Sternal ribs. (A, B, and C are after Giebel; D is after Flower.)

Fig. 357. - Osteology of Monotremes. A, Skull of Echidna hystrix. B, Side view of the skull of Ornithorhynchus anatinus, and C, lower jaw of the same, viewed from above, showing the horny dental plates (m). D, Sternum and adjacent parts of the skeleton of a young Ornithorhynchus: c c Clavicles; i Interclavicle; p Presternum ; ms, Mesostemum; r r Vertebral ribs; ir Intermediate ribs ; sr Sternal ribs. (A, B, and C are after Giebel; D is after Flower.)

The Ornithorhynchus or Duck-mole is one of the most extraordinary of Mammals. The body (fig. 358) resembles that of a mole or small otter, and is covered with a close, short, brown fur. The tail is broad and flattened. The jaws are produced to form a beak just like that of a duck in appearance: hence the name of "Duck-billed animal," often applied to it. The margins of the jaw are sheathed with horn, and are furnished with transverse horny plates, two in each jaw (fig. 357, B and C): but there are no true teeth. The sternum (fig. 357, D) is of five pieces, and there are sternal ribs. The nostrils are placed at the apex of the upper mandible. The legs are short, and the feet have five toes each, furnished with strong claws, which enable the animal to burrow with facility. The toes are also united by a membrane or web, so that the animal swims with great ease. The Ornithorhynchus is exclusively found in Australia, ranging as far north as Queensland, and inhabits streams and ponds. Its food consists chiefly, if not exclusively, of insects, and the animal makes very extensive burrows in the banks of the rivers which it frequents. The young are born quite blind, and nearly naked, and the method in which they obtain milk from the mother is somewhat obscure, as there are no nipples, nor is there any marsupial pouch. It is certain, however, that the beak of the young animal is extremely different from what it is in the adult condition. The young animal is totally hairless, the mandibles are soft and flexible, the tongue is not placed far back in the mouth (as it is in the adult), and the eye is at first covered by the skin.

Fig. 358.   Ornithorhynchus anatinus.

Fig. 358. - Ornithorhynchus anatinus.

The genus Echidna (Tachyglossus) is represented by four species - viz., E. hystrix, E. setosa, E. Lawesi, and . Bruijnii, the first being Australian, and the second Tasmanian, whilst the two last occur in New Guinea. The Echidna hystrix is the best-known species, and in some external respects is not unlike a large hedgehog, having the back covered with strong spines, interspersed with a general coating of bristly hairs. The snout has not the form of a duck's bill, as in the Ornithorhynchus, but the two jaws are greatly elongated, and are enclosed in a continuous skin till close upon their extremities, where there is a small aperture for the protrusion of a very long and flexible tongue. The jaws (fig. 357, A) are wholly devoid of teeth or anything in the place of teeth; and the nostrils are placed at the extremity of the cylindrical snout. The feet have five toes each, furnished with strong curved digging-claws, but the toes are not webbed. There are no nipples, but the ducts of the mammary glands open at the bottom of two integumentary pouches, which are said to be sufficiently capacious to hold the young when first born. The Echidna hystrix measures from fifteen to eighteen inches in length, and is a nocturnal animal. It lives in burrows and feeds upon insects, which it catches by protruding its long and sticky tongue.

As regards the distribution of the Monotremes in time, no fossil remains referable to the order have ever been discovered, with the exception of a gigantic Echidna, recorded by Mr Krefft as occurring in the Post-tertiary deposits of Australia.