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Solenogastres and Caudofoveata (Aplacophora) remain some of the least known molluscs, despite ubiquity in the marine environment and importance in understanding molluscan evolution. The use of new morphological techniques and development of DNA barcode libraries have helped make specimen identification easier. However, for solenogasters, using histology for identification and adequate description of species remains necessary in most cases. This, together with the facts that knowledge about solenogaster species distributions is biased and that most species were described from one or very few individuals, explains why many open questions about the actual distribution, intra- and interspecific variability, etc., remain. We performed an integrative taxonomic study of eight specimens of solenogasters from the South China Sea (West Pacific Ocean) thatresulted in the identification of four new species of Proneomeniidae. Species identification and description following the established diagnostic characters were straightforward. However, phylogenetic analysis of molecular data obtained from these specimens and other members of Proneomeniidae indicate that the family is polyphyletic. We recovered representatives of two other families, Epimeniidae (Epimenia) and Strophomeniidae (Anamenia), nested within Proneomeniidae with strong support. Ancestral character state reconstruction indicates that characters commonly used in solenogaster taxonomy, such as the radula and foregut glands, may be more evolutionarily labile in this group than previously known. Therefore our work fills knowledge gaps regarding the diversity and distribution of members of this family but raises important questions about solenogaster taxonomy and systematics that should be further assessed with additional markers and broader taxon sampling. ZooBank: urn:lsid:zoobank.org:pub:BCADACD6-9AD0-442A-AD64-031BA8D88599 

So far, of the 292 known species of solenogasters (Mollusca, Aplacophora), 62 belong to the clade Pholidoskepia Salvini-Plawen, 1978. Of these, only two have an abyssal distribution (3500–6000 m depth). Among Pholidoskepia, Dondersiidae Simroth, 1893 is the most diverse family. This study contributes to the knowledge of this family with the description of one new genus and six new species from the abyssal South Atlantic Ocean: Dondersia ? foraminosa sp. n., Nematomenia divae sp. n., Nematomenia brasiliensis sp. n., Nematomenia ? guineana sp. n., Helluoherpia vieiralaneroi sp. n. and Inopinatamenia (gen. n.) calamitosa sp. n. Specimens were collected during DIVA (Latitudinal Gradients of Deep-Sea BioDIVersity in the Atlantic Ocean) expeditions in the Guinea (DIVA 2 Me 63/2, 2005) and Brazil (DIVA 3 Me 79/1, 2008) Basins. Specimens were characterized based primarily on the sclerites and internal anatomy, which was studied using histology. The importance of the radula and mantle sclerites for taxonomy is emphasized. Amended diagnoses for the family and some genera within this family are provided. This contribution increases the described diversity of Dondersiidae to ten genera and 38 species and highlights the need for more study of solenogasters in the deep sea.