noun, plural: prototherian mammals
Any from the group of mammals in which the young develop within the egg that is laid down by oviparous mothers
Animals of Class Mammalia may be grouped as either prototherian or therian. Therian, which includes metatherians and eutherians, give birth to a live young whereas prototherian mammals are egg-laying animals, such as the monotremes.
Word origin: Greek proto (first) + therion (beast) + mammal