monotreme
mo·not·reme
noun
1.
the most primitive mammals comprising the only extant members of the subclass Prototheria
Origin
From mono- (“one”) + -treme (“hole”).
Related words
Broader terms
WordNet 2.1 (Princeton University). Wiktionary (CC BY-SA). WordNet 2.1 Copyright Princeton University. All rights reserved. Wiktionary content is available under the Creative Commons Attribution-ShareAlike Licence.