A significant part of the collection of mid- and Late Cretaceous (Cenomanian, Campanian and Maastrichtian) gastropods, bivalves and worms described and/or illustrated by de Ryckholt in his seminal work Mélanges paléontologiques between 1854 and 1862, has recently been retraced in the historical collections at Liège University. Of the original collection, more than 206 specimens, including 196 nominal types (lectotypes and genotypes), all considered lost, are now available. The genotypes of the gastropod genera Tudicula de Ryckholt, 1862a and Prosopostoma de Ryckholt, 1862a are photographically illustrated for the first time. Prosopostoma bucculans, from the Cenomanian Bernissart Formation (formerly ‘Tourtia de Tournai’), is here chosen as the type species of the genus Prosopostoma, a possible stromboid. We also reassess the stratigraphic age of the type localities under the revised stratigraphic framework of Belgium and a detailed account on the research history of this material, in order to provide a thorough scientific background for future study of this formidable collection.