"Something old, something new, something borrowed, and the oioxeny is true": description of n. sp. (Polyopisthocotylea, Plectanocotylidae) from the MNHN Helminthology collection with novel molecular and morphological data for . (Van Beneden & Hesse, 1863) () from Sweden.

Natural history museums worldwide house billions of apposite specimens, offering the potential for cost-free parasitological datasets. Herein, we provide novel morphological and molecular data (28S and 1) for the polyopisthocotylean from the type-host from Sweden based on newly collected specimens from the Northeast Atlantic, and specimen from T. Odhner's collections at the Swedish Museum of Natural History (Stockholm, Sweden). The newly generated 28S sequences of from from the Northeast Atlantic were identical to those from the Western Mediterranean, and nested in a single clade, suggesting the presence of a single species. A 28S sequences of from Sweden and those from the U.K. (type locality for ) were identical; we confirm that and are conspecific and formally synonymize them. A single 28S sequence of sp. from off France differed from from the Northeast Atlantic by 3-4 % and from from France by 3%. sp. ex off France is clearly not , suggesting an oioxenic specificity of to . Careful re-examination of cf from from the Western Mediterranean from the Helminthology collection of Muséum national d'Histoire naturelle (Paris, France) revealed that it differs from all congeners by morphometry (size of clamps, of terminal lappet and its hamuli and uncinuli, and size of atrial spines). The 1 divergences between . cf and , , and were 10-11 %, 10-11 % and 8 % respectively, falling within the interspecific variations range. from the Mediterranean is described as a new species, n. sp. We apprise nomenclature problems in and consider a .