Paleontologists using thin sections or acetate replicas of colonial skeletons can standardize their biometric procedures by using the methods of quantitative stereology, thereby removing errors, inconsistencies, and redundancies, and enriching their pool of available morphological information. From three fundamental quantities, counted as interceptions of structures with randomly oriented test grids, lines, and areas, seven useful derived quantities can be calculated using simple formulas. Stereology allows the discrimination of two kinds of constructional gradients in stenolaemate bryozoans. In a case study of the Ordovician trepostome Amplexopora filiasa, structural variation reflects both an astogenetic trajectory as well as phenotypic deviations from it, which lead (in one direction) to surfaces of minimum area caused by close packing. Inflecting acanthostyles are inferred to increase intrazooidal surface area (as measured by perimeter length) as a compensation for surface area loss in close packing, thus maintaining a developmental buffering system.--Modified journal abstract.