The Megara Basin in Greece, located east of the Corinthian Gulf and bordered by the Alkyonides Gulf to the north and the Saronic Gulf to the south, comprises Neogene-Quaternary formations rich in marine and lacustrine fossil mollusks. The shell morphology of T. micans closely resembles that of both fossil and extant taxa, notably Theodoxus meridionalis, a species that is widely distributed throughout the Mediterranean region, including Greece. However, substantial intraspecific variability in shell shape and coloration among Theodoxus species complicates taxonomic assignments. Conversely, the operculum has been recognized as a potentially more reliable feature for species-level differentiation. Our research draws on material from the historical collection of the Museum of Geology and Paleontology at the National and Kapodistrian University of Athens (NKUA), which includes fossil specimens and sediment samples from sites such as Alepochori and the Mauratza Ravine. This material consists of both bulk fossiliferous sediments and previously sorted fossils from the 20th-century research. We discovered opercula, which were identified as belonging to T. micans. These were found alongside T. micans shells and other species such as Lymnaea megarensis Gaudry, 1867, but no other Theodoxus species. Comparative analysis with extant and fossil material from other localities—where T. micans has been reported—yielded no prior operculum descriptions. Our findings suggest that T. micans may be conspecific with or closely related to T. meridionalis. These results contribute to the ongoing taxonomic and systematic reassessment of this fossil taxon.