ornithodelphiaThe lowest one of three subclasses of the class Mammalia, represented by the monotremes or oviparous mammals, and conterminous with the order Monotremata: so called from the ornithic character of the reproductive or urogenital organs. These mammals lay eggs, like birds; the separate oviducts open into a cloaca common to the genital, urinary, and digestive organs; the vasa deferentia of the male open also into the cloaca; and the testes are abdominal. The mammary glands are nippleless. The sternum has a peculiar tau-bone or T-shaped interclavicle (see cut under interclavicle), and the coracoids articulate with the sternum. The superior transverse commissure of the brain has no well-defined psalterial fibers, and the septum is much reduced in size. The Ornithodelphia are also called Prototheria.
Chambers's Twentieth Century Dictionary
Ornithodelphiathe lowest of the three sub-classes of mammals, same as Monotremata—from the ornithic character of the urogenital organs