Let bygones be bygones: The troubled chronicle of Odostomia unidentata (Montagu, 1803) (Gastropoda: Pyramidellidae)

The recent publication by Oliver and co-authors about the extant type material of the molluscan taxa described by George Montagu has revealed that the only existing syntype of Turbo unidentatus Montagu, 1803, currently accepted as Odostomia unidentata, does not fit the current concept of this species but is a gerontic specimen of Brachystomia eulimoides (Hanley, 1844) instead. In order to understand the origin of this inconsistency, we studied the variability of the two species and carried out a review of the nomenclatural history of Odostomia unidentata. We examined a large amount of material to show that, apart from the different type of protoconch, a feature often neglected by ancient authors, the two species clearly differ due to the larger size reached by Brachystomia eulimoides and the different proportions, which are maintained in gerontics. A survey of the ancient literature showed that there was no consensus among 19th century authors about the identity of Montagu’s species. We reconstructed the troubled nomenclatural chronicle of this species and showed that, with very few exceptions, most authors misinterpreted Montagu’s description inverting the two species, until the current usage of Odostomia unidentata was widely accepted following Jeffreys’ (wrong) interpretation. The different ecology of the two species further supports this picture. To preserve nomenclatural stability, we are preparing an appeal to the International Commission on Zoological Nomenclature to use its plenary powers to set aside the only syntype of Turbo unidentatus and designate a neotype, based on a specimen fitting the current concept of O. unidentata, in agreement with Article 75.6, in order to maintain the prevailing usage of Montagu’s taxon.