allantoidian,a.andn.Zool.|ælænˈtɔɪdɪən| [ad.Fr.allantoïdien: see prec. and -ian.] A.adj. Having the fœtus furnished with an allantois. B.n. An animal whose fœtus is so furnished. 1861Hulme Moquin-Tandon's Med.Zool. ii. ii. 62 Allantoidians = Mammalia, Aves, Reptilia.