Shells of Lingula squamiformis from argillaceous sediments at three horizons within the Dinantian Series, exposed at three localities in Scotland, have been studied to determine chemico-structural changes resulting from fossilization. Biomineral structures are essentially the same as those of living Lingula anatina with apatitic granules aggregating into spherules, up to 60 nm in size, and larger spheroidal bodies as well as rods and rarer lath-like plates. These aggregates and the original organic constituents were secreted as stratiform successions in two distinct layers, as in Recent L. anatina. The outer, lithified part of the primary layer bears microstructural moulds of a totally degraded periostracum and was probably composed mainly of acidic glycosaminoglycans (GAGs); the inner part evidently contained a higher proportion of spherular apatite within the GAGs than in living species. The secondary layer consists of variably complete rhythmic sets of compact, rod and plate (virgose), and membranous laminae. Compact laminae are normally cleaved along degraded walls of GAGs whereas the original GAGs-filled spaces of virgose laminae are partly occupied by recrystallized apatitic sheets with kaolinite. The phosphatized membranous laminae probably contained more spherular apatite in life than present day Lingula. The shell is canaliculate with chambers and galleries well developed in the virgose laminae. There is a decrease in concentration of amino acids from the posterior to the anterior of the valves of living Lingula anatina and Glottidia pyramidata resulting from the proteinaceous coat of the apatitic spherules. A similar distribution of hydroxyproline occurs, indicative of collagen in the body platform of living lingulids. Nearly all organic constituents have been degraded in the Carboniferous valves but threads, about 50 nm thick, occasionally traverse spaces in virgose laminae and even form a network coated with spherular apatite, which resembles webs of collagens or actin found in living lingulids. Acidic and aliphatic amino acids were extracted from L. squamiformis valves from Calderwood and Kinghorn whereas the narrower range of amino acids from the heavily pyritized valves from Ardross confirmed differential degradation of organic material during the fossilization of penecontemporaneous samples. The fossilization of complete shells of L. squamiformis is not due exclusively to catastrophic burial as has been deemed necessary to preserve Recent Lingula intact. The living shells of Carboniferous species were more apatitic than those of Recent Lingula, especially in the anteriomedian sectors of the secondary layers.