What is the true name for european populations of the slug Arion lusitanicus?

The name Arion lusitanicus can no longer be applied to this taxon. A recent re-description of lusitanicus sensu Mabille, based on topotypes from Setúbal, Serra da Arrábida, Portugal (Castillejo 1997) has shown that Portuguese lusitanicus has a very different spermatophore and internal morphology from the central and west European species to which this name was applied, first by Altena (1956) and then by subsequent authors. Falkner et al. (2002) have proposed that the north-west European species be called vulgaris Moquin-Tandon, 1855 as this is the first name which can be unambiguously applied. British authors including Quick (1960) confused vulgaris with Arion flagellus Collinge. Some of Quick’s drawings of ‘lusitanicus’ (= vulgaris), particularly of the spermatophore, actually relate to flagellus. The reader is directed to Davies’ (1987) paper which first resolved the confusion here and gives a detailed account of the distinguishing features and reproductive biology of both species.

References
ALTENA C. O. VAN REGTEREN 1956 Notes sur les limaces, 3. (Avec la collaboration de Mm. D. Atenet A. R. Schouten) Sur la présence en France de’Arion lusitanicus Mabille Journal de Conchyliologie
95: 89-99.
CASTILLEJO J. 1997. Babosas del Noroeste Ibérico Universidade de Santiago de Compostela, Spain. 192pp.
FALKNER G., RIPKEN T. E. J. & FALKNER M. 2002. Mollusques continentaux de France Liste de Référence annotée et Bibliographie. Patrimoines Naturels, 52, Museum d’Histoire Naturelle, Paris. 350pp.
QUICK H.E. 1960 British slugs (Pulmonata: Testacellidae, Arionidae, Limacidae) Bulletin of the British Museum (Natural History) Zoology 6: 1-226.