Previous investigators interpreted stromatoporoids to have been composed originally of aragonite; however, petrographic observations of stromatoporoid microstructure indicate that those within the Devonian Helderberg strata of New York State were originally composed of high-magnesian calcite. The delicate reticulate structure of pillars and growth laminations of the original stromatoporoid coenosteum is represented by a neomorphic fabric of microdolomite-rich zones of inclusions in low-magnesian calcite. In comparison, co-existing constituents that were originally composed of low-magnesian calcite (e.g., brachiopods) display 1) better preservation of their ultrastructure than do the stromatoporoids, and 2) an absence of microdolomite. Gastropods, originally composed of aragonite, are all represented by micrite envelopes encompassing pore-filling spar accumulations, i.e., complete loss of ultrastructure. The style of preservation of the stromatoporoids is essentially identical to that of associated echinodermal debris, an original high-magnesian constituent. Both the stromatoporoids and echinoderms are preserved as microdolomite-bearing diagenetic low-magnesian calcite. Consequently, these stromatoporoids were originally composed of high-magnesian calcites.