1. The thin fragments of leaflets resulting from ultrasonic irradiation of the organic membranes of conchiolin freed by decalcification of mother-of-pearl by chelating agents, have been studied with the electron microscope in six species of Gastropods and in 30 species of Pelecypods. The material includes shells found in archaeological excavations and subfossil species. 2. In all the specimens, the fragments appear in the shape of lacelike reticulated sheets, perforated by openings or pores separated by cylindrical or strand-like trabeculae. 3. The average diameter, the frequency, and the relative surface of the pores in the reticulated sheets, have been measured. Previous conclusions about statistically significant differences in the patterns at the class level between Gastropods and Pelecypods have been supported by the present observations. In Pelecypods, the present results have brought evidence of significant differences between the patterns at a subclass ( family ) level. 4. The successive steps of disruption of the reticulated sheets and of their trabeculae by protracted ultrasonic irradiation have been observed in several species (including Trigonia lamarckii, Grandidieria burtoni and Aetheria elliptica) . 5. The findings suggest that the trabeculae consist of a core of fibrils, coated in normal conditions by sheets or muffs of another material which appears in part in the shape of hemispherical protuberances of various sizes. The similarity of these mechanical effects of ultrasonic irradiation with modifications induced in previous studies in the organic reticulated sheets of mother-of-pearl by biochemical fractionation is discussed. It is assumed that differences in the size and in the arrangement of the fibrils forming the core of the trabeculae, and that variations in the spatial relations between fibrils and coating substances might account for the taxonomical differences recorded between species with regard to the pattern of the organic lace-like reticulated sheets
