Monotremata (Gr. . , single, and , opening), an order of implacental mammals; the name is derived from the fact that the intestinal, generative, and urinary organs open into a common cloaca, as in birds and reptiles. The order includes the ornithorhynchus and the porcupine ant-eaters (echidna), from Australia and Tasmania. They are the lowest mammals, and have many characters of birds and reptiles in their structure and mode of reproduction; they have no abdominal pouch, but the marsupial bones are present; at the top of the breast bone is an episternum with lateral arms forming the chief support of the scapular arch, on the top of which the true clavicles, like the furcular bone of birds, are situated; the coracoid bones extend also to the sternum, and are surmounted by epicoracoid bones; in the scapular arch, therefore, they resemble in some respects birds, in others lizards and enaliosaurians. The eyes are very small, the external ears absent, and the face projects in the form of a naked beak, without teeth or soft movable lips; the teeth are replaced by small horny plates; some authors, as Wagner, have ranked them among edentates.
The feet are five-toed, with long nails; the males have a long spur on the hind legs, the groove of which communicates with a glandular organ, whose secretion has been erroneously supposed to be poisonous; the mammary orifices are mere slits in the skin of the abdomen, without elevated nipples, and the female sexual organs resemble those of birds; the young are born alive, and are suckled as in other mammals; the caecum is small; the lungs are spongy and cellular, and enclosed in a thoracic cavity separated from the abdomen by a diaphragm; in the brain there is no corpus callosum, and the bigeminal bodies are simple. (See Ornitiiorhynciius, and Porcupine Ant-Eater).