Nomenclatural and taxonomic notes on two species of ribbon worms (Nemertea) collected during the La Recherche Expedition (1838–1840)

Illustrations of at least three unidentified species of nemerteans from the Nordic Seas, collected during the French La Recherche Expedition (1838–1840), were included in a work published around 1842–1848 as part of a series reporting the results of the expedition. Of these three species, two were later named based on the illustrations: Meckelia borealis Diesing, 1862 and Ditactorrhochma typicum Diesing, 1862, which were subsequently reclassified in 1904 as Cerebratulus borealis (Diesing, 1862) and Amphiporus typicus (Diesing, 1862), respectively, and have been accepted as valid to date. The genus Ditactorrhochma Diesing, 1862, established for D. typicum, has been synonymized with Amphiporus Ehrenberg, 1831 since 1904. In this article, the lineid heteronemertean Cerebratulus borealis is deemed a nomen dubium because it cannot be distinguished with certainty from several similar existing species, including those with higher priority, namely Lineus ruber (Müller, 1774) and L. viridis (Müller, 1774). Meanwhile, the monostiliferous hoplonemertean Amphiporus typicus is identified herein as a junior subjective synonym of Nipponnemertes pulchra (Johnston, 1837). Ditactorrhochma Diesing, 1862 is thus a senior subjective synonym of Nipponnemertes Gibson & Crandall, 1989. Ditactorrhochma has not been used as a valid name since 1900, whereas Nipponnemertes has been used prevalently, meeting the conditions stipulated in Article 23.9.1 of the International Code of Zoological Nomenclature (fourth edition). Therefore, Ditactorrhochma Diesing, 1862 is now declared a nomen oblitum with respect to Nipponnemertes Gibson & Crandall, 1989, the latter being a nomen protectum.