One of the four subclasses of the class Mammalia. Prototheria contains a single order, the Monotremata. No ancestral genera of fossil monotremes are known, and the structure of the living monotremes is so specialized that the affinities of the Prototheria are largely conjectural. Most mammalogists believe that the prototheres arose from a different stock of therapsid reptiles than the one that gave rise to the Theria.