The genus Ilyanthus was erected by Forbes (1840) for I. scoticus; little is known about that species, and its anatomy is undescribed. There is no evidence that any subsequently described species is really a genuine Ilyanthus, and the genus-name can only be used provisionally for others until I. scoticus can be dissected. Meanwhile the genus-name must be reserved for I. mitchellii, the main subject of this paper. Gosse described this species from a specimen which he saw in 1853 (see Actinologia, p. 232); it must have been in poor condition or semi-contracted, and his figure (PL. 8, Fig. 6) is quite misleading. Andres erected (1883, p. 462) a genus Mesacmœa for his Ilyanthus stellatus of 1880, and, as I hope to show below, M. stellata is either specifically or at any rate generically identical with I. mitchellii; so that Mesacmœa now becomes a synonym; if it should prove, later on, when I. scoticus can be examined, that I. mitchellii does not really agree with it, i.e. is no Ilyanthus, then the name Mesacmœa can be revived for mitchellii and stellatus. Carlgren (Actiniaria of the Danish Ingolf—Expedition, 1921) has given a few anatomical details of Me acmœa, gained from notes by Andres in his possession; they agree with my description of I. mitchellii as far as they go, but there are not enough of them to make a full understanding of the genus possible. In Andres' 1883 monograph will be found a description of another species, I. partenopeus (p. 459).