Restudy of Clarke's type material for Estheria ortoni brought to light an overlooked valve feature of critical taxonomic importance. This feature is the recurved posterior valve margin. This characteristic combined with that of a tubercle or spine on the initial valve in the same individual gives 2 distinct conchostracan shell features. These 2 features are the pemphicyclid (bearing an umbonal spine) and the limnadiopsid (with posterior recurvature of growth lines). This combination of valve features is the basis for the establishment of a new family, Pemphilimnadiopseidae, which includes Clarke's type material of E. ortoni. The tubercle or node that Clarke reported was found to be either a spine base or a hollow spine. The ontogenetic development of Estheria ortoni indicates that the pemphicyclid condition is ancestral to the paleolimnadiopsid condition. This observation must be reconciled with the older occurrence in the lower Mississippian of Paleolimnadiopsis jonesi, which has posterior recurvature but no umbonal spine. Either a pre-Mississippian pemphicyclid ancestor, presently unknown, gave rise to 2 distinct lines, the ortoni line and the jonesi line, or posterior recurvature of growth lines occurred independently in each of these lines.