One of the mysteries of the English language finally explained.
Treated as plural. A subclass or infraclass of mammals including the monotremes and sometimes also the extinct multituberculates.
Mid 19th century; earliest use found in Thomas Huxley (1825–1895), biologist and science educationist. From scientific Latin Ornithodelphia, subclass name (H. M. D. de Blainville) from ornitho- + ancient Greek δελϕύς womb + scientific Latin -ia, so named on account of the resemblance of the reproductive organs to those of birds.
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