AbstractCytoarchitecturally defined cortical areas of the posterior cingulate gyrus differ morphometrically among the major taxonomic divisions of primates. Prosimians and anthropoids have different laminar proportions in the isocortical regions (areas 30, 23, and 31); anthropoids have relatively larger outer main laminae and granular layers than prosimians. In the granular retrosplenial cortex (area 29), however, prosimians and anthropoids differ only in the proportion of the molecular layer to the rest of the cortex. On the other hand, the proportions of the inner and outer main laminae of area 29 differ between the infraorder divisions of Anthropoidea, the catarrhines, and the platyrrhines. The platyrrhines (New World monkeys) have apparently specialized by increasing the outer main lamina of area 29, which contains afferent and intracortical connections. Among all primates, the amount of neuropil in each cortical region changes as a function of brain weight, but within each area, the neuropil maintains a constant ratio between the outer and inner laminae. These observations suggest that neuropil ratios are conservative features in primates and that evolutionary changes more frequently involve shifts in volumetric proportions. Furthermore, the platyrrhine‐catarrhine differences in area 29 likely evolved after the prosimian‐anthropoid differences were established in the isocortical cingulate regions.