Accurate species recognition is an essential part of biological study. In practice, morphology is most used to differentiate species, with the assumption that species-level genetic differences are reflected morphologically for modern and fossil specimens. However, this assumption may not apply in cases of “cryptic species”, genetically distinct but morphologically indistinguishable species, and when morphological variation within species is difficult to delineate from species-level differences. While taxon-specific practices and biological considerations are interconnected in cryptic species study, most extant shelled marine gastropods were found not to be morphologically cryptic, and cryptic species were reported with variable confidence. This result suggests that a similar proportion of gastropod species may be identifiable as fossils. To further examine conchological-based species recognition and the concordance between genetic and conchological differences, several species in the well-known common intertidal gastropod, Littorina (periwinkles, Littorinidae) were investigated using landmark-based morphometrics on shell shape and genetic data. Genetic and conchological differences were positively correlated overall in the Littorina examined (planktotrophic developing East Pacific †L. petricola, L. keenae, sister species L. plena and L. scutulata; North Atlantic L. littorea), except for L. obtusata (no correlation; North Atlantic direct developer), giving confidence to conchological-based species identification. Distantly related Littorina species were more reliably recognized conchologically than closely related species. Genetic and shell shape differences were not correlated with spatial distance for either planktotrophic or direct developing species studied, contrary to expectations based on development. A comparable amount of error was found for shell-based species classification in modern and fossil individuals of the same species. This study draws attention to the interrelatedness of scientific practice and interpretation of results, as assumptions should be made clear, and evaluated where possible.