Prototheria
A taxonomic subclass of mammals that includes monotremes (egg-laying mammals). Used in biological and zoological contexts.
Prototheria includes the platypus and echidnas.
単孔類 (monotremes) is the common Japanese term for the living members of Prototheria, while 原獣亜綱 is the formal taxonomic subclass name.
From 原 (primitive) + 獣 (beast) + 亜綱 (subclass), a direct translation of the taxonomic term Prototheria.