(Diagnosis corresponding to that of the genus Iasis, since only a single species has been recognized).
Solitary zooids: up to 100 mm long (Iasis zonaria, Iasis zonaria 1). Test very firm with a longer postero-median projection. Five body muscles. Intermediate muscle and body muscles broad. All the muscles interrupted dorsally and ventrally. The mature stolon describes one turn around the nucleus.
Aggregate zooids: test very firm, asymmetric, with a posterior projection on its right side (Iasis zonaria 2). Five body muscles, all of them interrupted ventrally, MI interrupted dorsally as well; MV divided into two branches on the right side. Testis projecting as lobules around the nucleus. Up to 7 embryos between MIV and MV (Esnal et al., 1987).