Type genus: Isis Linnaeus, 1758

Diagnosis. Colony with articulated skeleton of gorgonin nodes alternating with calcareous internodes that are solid and composed of radiating bundles of sclerite-like units. Branches originate from the internodes, always beginning with a node at the base of the branch. Coenenchyme thick; polyps completely retractile into small pockets in the coenenchyme, leaving only small pores visible on the surface. Polyps mostly devoid of sclerites, with small rods only in the tentacles. Coenenchyme sclerites mostly small clubs, but may include 6-, 7- or 8-radiates, warty rods, spindles and crosses, heavily sculpted capstans, dumbbells, and double cones.

Included genera: Isis Linnaeus, 1758

Remarks. The genus Chelidonisis has routinely been included in the Isididae subfamily Isidinae with Isis, chiefly on the basis of the presence of 6-radiate sclerites in the coenenchyme. However, there are several morphological differences that distinguish the two genera. The axis of Isis is thick with smooth longitudinal ridges on the internodes, whereas in Chelidonisis the axis is slender and the ridges on the internodes are festooned with small spines. The coenenchyme of Isis is very thick and encompasses retractile polyps as compared to the thin coenenchyme of Chelidonisis, wherein the polyps contract into armored verrucae on the branch. In addition, the coenenchyme sclerites of Isis are very diverse capstans, tuberculate spindles, and clubs, whereas in Chelidonisis the sclerites are predominantly 6-radiates and spindles, and capstans are never present. Finally, the two genera occur in widely separated clades on the molecular phylogeny (Figures 2, 3), as also shown by Quattrini et al. (2020).