Prototheria (próto-thiŕ ee-a) (Gr.prōtos, first, + thēr, wild animal) is the subclass to which the order Monotremata belongs.
Most of the animals in this subclass came from the Cretaceous and early Cenozoic period, which are all extinct now except for the egg laying monotremes. One such animal is the Platypus.
Since Monotremata is the only order currently in the subclass Prototheria, it seems somewhat redundant. However, it is always possible that fossils will come to light placing creatures of some other order in that group, and Prototheria is often used in contrast to the two other subclasses, Metatheria and Eutheria. These three subclasses (Prototheria, Metatheria, and Eutheria) mean "first beast", "intermediary beast", and "true beast". The names were first proposed by Thomas Huxley. Metatheria and Eutheria are often grouped as magnorders of the subclass Theria.