Monotrem'ata (Gr. monos, 'single;' trēma, 'an opening'), the lowest order of mammalia, in many of their characteristic points indicate an approximation to reptiles. The skull is smooth; the brain-case very small as compared to the face; the snout much prolonged, and the jaws unprovided with soft movable lips, and not furnished with teeth, except in the young Ornithorhynchus, where they have been discovered by Poulton and Thomas. The cranial bones coalesce, as do a bird's, at a very early period, and leave no signs of sutures. The external ear is altogether absent; while the eyes, though small, are perfectly developed.
The bones of the shoulder, forming the scapular arch, are unlike those of any other mammals, and resemble those of reptiles. At the top of the sternum is a T-shaped bone, formed by the union of the two clavicles, corresponding to the furculum in the bird's skeleton, and to the clavicles and interclavicle of the reptile. The coracoid bones, which in other mammals are mere processes of the scapula, are here extremely large, and assist in strengthening the scapular arch; they are produced beyond the socket of the humerus (the glenoid cavity), so as to articulate with the sternum. The pelvis is provided with marsupial bones. The ovaries are analogous to those of the Sauropsida (reptiles and birds), the right ovary being comparatively undeveloped, while the left forms a racemiform mass. The orifices of the urinary canals, the intestinal canal, and the generative canal open, as in birds and reptiles, into a common cloaca, from which circumstance the order Monotremata derives its name. The ova (as has been shown by Poulton for Ornithorhynchus, and by Beddard and Caldwell for Echidna) are of large size, and contain an immense amount of yolk, as in the reptile or bird. Caldwell has discovered also the important fact that the early stages of development are like those of a reptile, and has confirmed the earlier discovery that these mammals lay eggs furnished with a thick shell. The Echidna carries its eggs in its pouch, but the Ornithorhynchus deposits them in its burrow. The mammary glands, of which there is only one on each side, are not provided with nipples, but open by simple slits on each side of the abdomen. It has been proved, moreover, that the mammary glands are altogether different from those of other mammals, and only functionally resemble them.
This order includes only two or three species, all natives of Australia or Van Diemen's Land, which, however, form two families—the Ornithorhynchidae (see ORNITHORHYNCHUS) and the Echidnidae (see ECHIDNA). It appears probable from what is now known of the teeth of Ornithorhynchus that some of the Mesozoic mammalian remains which were formerly referred to the Marsupialia are really those of Monotremata. The literature of the group is fully referred to in The Catalogue of Marsupialia and Monotremata (British Museum).