'Monotreme' definitions:

Definition of 'monotreme'

noun

The most primitive mammals comprising the only extant members of the subclass Prototheria [syn: monotreme, egg-laying mammal]

From: GCIDE

Monotreme \Mon"o*treme\, n. [Cf. F. monotr[`e]me.] (Zool.) One of the Monotremata. [1913 Webster]