Crossword clue: the most primitive mammals comprising the only extant members of the subclass prototheria

The most common answer is monotreme (9 letters).

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

Possible answers

monotreme 9 letters wn-def

the most primitive mammals comprising the only extant members of the subclass Prototheria