Prototheria is the subclass to which the order Monotremata belongs. Since Monotremata is the only order in the subclass Prototheria, it is somewhat redundant. However, Prototheria is often used in contrast to the two other subclasses, Marsupialia and Placentalia (or, in more modern scientific terms, Metatheria and Eutheria). These three subclasses, Prototheria, Metatheria, and Eutheria, mean 'first beast', 'intermediary beast', and 'true beast', first proposed by Thomas Huxley. Metatheria and Eutheria are often grouped as magnorders of the subclass Theria.
