The taxonomy of conchostracans bearing an umbonal spine, a larval feature, is reviewed. The family Vertexiidae is shown to embrace the genera Echinestheria, Gabonestheria, (reassigned here to Cornia), Vertexia, and Curvacornutus, n. gen. The original figures of types are reproduced. Raymond's types of Pemphicyclus are restudied. The "tubercle" is found to be a "spine," adult valves are found to bear a flattered flare, and general morphology is similar to specimens lacking umbonal spines. The latter specimens are found in older Wellington beds. The possibility of dimorphism in Gabonestheria is considered. Isaura (Euestheria) harveyi, Form A, based on a restudy of ornamentations, is reassigned to the subgenus Lioestheria, since all evidence of polygonal sculpture is lacking. The one new genus, Curvacornutus, and the 3 new species C. prima, C. modesta and Gabonestheria dickinsoni are described and illustrated from the Kansas Wellington. The inferred evolutionary significance of larval shell spinosity and the condition of dorsal margin serration is discussed.