A sample of 100 Spiriferina stracheyi (Salter) from a single chronodeme of the Anisian Whitehorse formation, Alberta Rocky Mountains, is the subject of covariation studies (individual allomorphosis) in the parameters half width + length mean ("size"), half width/length ratio, thickness, apical angle of fold and sulcus boundaries (fold–sulcus angle), plicae frequency, and apical angle between second right and left plicae from the median (plicae angle). Changes of growth rates for half width/length and plicae frequency at a mean "size" of 10 mm suggest the attainment of maturity; parameters vary nearly independently from "size" in the sample of mature specimens. The variation in the parameters is unusually great. Significant correlation of −0.36 exists between half width/length and plicae angle, i.e. the plicae are more widely spaced in long ("rounded") valves than in wide ("alate") valve variants. A corresponding correlation of +0.61 exists for plicae frequency, wide valves averaging double as many plicae as narrow valves. The same correlations are present in a Himalayan sample of this species.Although, usually, shell plication of bivalved filter-feeders with accretionary growth may simply result from ecologic–physiologic functions of a plicate commissure, in the species here studied the plicae themselves might have had a mechanical function, i.e. the shell forming a corrugated membrane surface in which the "wavelength" increases with the axial growth of the membrane.