The genus—initially believed to be intermediate between Mesochra Boeck, 1865 (Canthocamptidae) and Cletodes Brady, 1872 (Cletodidae)—was established by Scott (1902: 461) to accommodate the type species P. longifurcata T. Scott, 1902 (by monotypy) from Loch Etive on the west coast of Scotland. Unaware of this description Sars (1906: 191) proposed the genus Stenheliopsis in the Diosaccidae (= Miraciidae) for its type and only species S. divaricata Sars, 1906 from the south and west coasts of Norway, and subsequently added three new species, S. latifurca Sars, 1911, S. media Sars, 1911 and S. affinis Sars, 1920, all from southern Norway (Sars 1911, 1920). Prior to these contributions Scott & Scott (1901: 343) had already described the morphologically similar species, Jonesiella brucei Scott & Scott, 1901, from east of Hopen, Svalbard Archipelago. Brady (1880: 31) had placed Jonesiella Brady, 1880 (containing the species J. fusiformis Brady, 1880 and J. spinulosa Brady, 1880) in the subfamily Stenheliinae. Sars (1909: 328, 336) considered Jonesiella a junior subjective synonym of Danielssenia in the Tachidiidae, with J. brucei being treated as a valid member of this genus. Note that as a result of reconsidering the status of Danielssenia fusiformis (Brady, 1880) nec Sars (1910) (the type species of Jonesiella) Jonesiella has now been reinstated as a valid genus in the Danielsseniinae (then Paranannopidae) (Huys & Gee 1993: 62).
Monard (1927: 149, 159, 164) maintained Stenheliopsis in the Diosaccidae, placed Pseudomesochra in the Canthocamptidae, and considered both Jonesiella and Danielssenia as valid members (without listed species) of the Tachidiidae. In a later report (Monard 1935: 16) it transpired that the author intended to restrict Jonesiella to include only J. brucei, a course of action that is in violation with the Code (ICZN Art. 67.2). Gurney (1932: 50) expressed the dissenting view that Pseudomesochra should probably be assigned to the Cletodidae. The author (Gurney 1932: 49) also proposed the new replacement name Nannomesochra for the junior homonym Pseudomesochra Gurney, 1927 which was introduced for a new genus of Canthocamptidae (Gurney 1927: 542).
Having been associated with four different families Lang (1936b: 174; 1936c: 447–448) finally resolved the taxonomic mess surrounding Pseudomesochra by relegating Stenheliopsis to a junior subjective synonym of the latter and placing the genus in the Diosaccidae (= Miraciidae), therefore endorsing Sars’s (1906: 192) earlier opinion that it is most closely related to Stenhelia Boeck, 1865. He also considered Jonesiella brucei a valid species of Pseudomesochra.
The familial assignment of the genus remained unchallenged until Willen (1996) transferred it from the Miraciidae as the type of a new subfamily Pseudomesochrinae in the Paranannopidae. Huys et al. (1996: 236), building on the non-availability of the generic name Paranannopus Lang, 1936d [nomen nudum], introduced the new family name Danielsseniidae to replace Paranannopidae [nomen nudum], since family-group names taking their stem from an unavailable (and thus invalid) generic name, also become unavailable (ICZN Arts 11.7.1.1 and 63–64). The family Paranannopidae was relegated to a subfamily of the Pseudotachidiidae by Willen (1999); hence at this rank it should be cited as Danielsseniinae Huys and Gee in Huys et al. (1996) (cf. Huys 2009: 11). Willen (2000) maintained the Pseudomesochrinae as one of four subfamilies recognized in her new classification of the Pseudotachidiidae.A second genus, Keraia Willen & Dittmar, 2009 was added to the Pseudomesochrinae (Willen & Dittmar 2009). The type genus Pseudomesochra has seen the addition of many new taxa since its proposal by Scott (1902) and currently includes 19 species (Table 2). Both P. perplexa Bodin, 1968 and P. gemina Coull, 1973 have been regarded as species incertae sedis by some authors (Willen 1996; Wells 2007) but are recognized as valid here.