ABSTRACT: The evolution of Lithastrinus from Eprolithus is observed in Turonian through Santonian material via variation in the number and morphology of wall elements. Previously, the shape of projections off the wall elements (rays) the size of the diaphragm, and inclination and twisting about the central area of the wall elements have been used to separate the genera, but these criteria are too general to consistently separate transitional 7-rayed forms. The more reliable criteria we have defined for distinguishing two different taxa, Eprolithus moratus and Lithastrinus septenarius, provides a more accurate biostratigraphic placement for these intermediate forms. The younger late Turonian to middle Santonian form, L. septenarius, is separated from the older form, E. moratus, through reduced central depression size (<50% the width of the entire central area) and rays that are generally longer (>50% the width of the central area) and angled away from the ring of elements (>15°). In Lithastrinus the rays only project from the proximal and distal sides of the wall elements, while in E. moratus they extend as a single ridge between the two sides. When in the focal plane of the diaphragm the wall elements appear as a rounded collar in L. septenarius, which can also be used to differentiate between the two taxa, especially in poorly preserved material.