Sagmatocythere rectum (Brady) (Loxoconchidae) comb. nov.—A case of homeomorphy ? Sagmatocythere rectum (Brady, 1868) is a Recent (presumed living) rare species, whose original description, based on the examination of only one carapace, was brief and without illustration.The analysed specimen was from Lerwick Bay, Shetland, dredged from a depth of c. 21–25 m, and the species was first attributed to the genus Cytheropteron, subfamily Cytheropterinae. Later, the species was found in other localities (west of Ireland, Bay of Biscay), and figured with some drawings in Brady & Robertson (1869, pl. XX, figs 6–8) and in Brady & Norman (1889, pl. XIX, figs 13, 14). Brady & Norman (1889) considered the species as belonging to the genus Bythocythere Sars, 1866, family Bythocytheridae. Athersuch & Horne (1983), studying specimens of Nannocythere pavo (Malcomson, 1886) and of Cytheropteron rectum Brady, 1868, two species considered by Brady & Norman (1889) as only one, with N. pavo being the juveniles of C. rectum, moved both species, completely distinct, to the family Loxoconchidae. Our attention was initially drawn to this species by its resemblance to members of the genus Microceratina, however, closer examination and comparison with Microceratina species especially from the Mediterranean demonstrated that we are dealing with an example of homeomorphy that merits clarification. The illustrations in Brady & Norman (1889), together with SEM images of one specimen from St Magnus Bay, Shetland (Norman Collection 1911.11.8 M3820), allow us to identify some Portuguese material (loose valves) collected in modern sediments of the Eastern Algarve continental shelf, off Tavira, and of the Mira estuary, SW Portugal, as belonging to the same species. The Portuguese specimens have a gongylodont hinge, NPC of two types and StPC (description below), and are considered as Loxoconchidae, in accordance with Athersuch & Horne (1983). We tentatively place the rectum species in the genus Sagmatocythere, described by Athersuch (1976), due to its dimensions> 0.35 mm (L = 0.43–0.45 mm), the sub-quadrangular to sub-rectangular outline, the pitted/reticulate ornamentation, the presence of few and simple marginal pore canals and of the gongylodont hinge with the terminal elements “comma-shaped” and lobate. However, the genus was described using morphological features of the valves and soft parts, the latter not available in the Portuguese material with only loose valves found. The Loxoconchidae genus Nannocythere Schäfer, 1953 possesses some characteristics in common with rectum species but has a smaller size (Microceratina, is presented below. The carapace is small, sub-rectangular in lateral outline. L S. rectum are never located in the cells. Such StPC have a diameter of about 3.2–3.5 µm with the central pore having a diameter of 1–1.5 µm. The sensillum emerging from inside the pore is slender, e.g., those from Fig. 15F have a diameter of about 0.5 µm. The area around the central pore is perforated with 10–15 small pores opening within tubuli which penetrate the valve. There is one, sometimes two rows of such pores within the area surrounding the central pore (Figs 15B, C, F). These minute pores have a size of: M = 0.3 µm (r = 0.18–0.43, N = 11). Internally the selvage of the RV extends up to the OM of the valve (Fig. 15P). The diameter of pores apparently emerging from a StPC (Fig. 15Q) is 3.33 µm; two tubules with a diameter of about 0.6–0.8 µm are visible inside the larger cavity. Other pores on the internal side of the valve are visible in Figs 15T, V, W; their diameter is approximately 5 µm. Hinge gongylodont (Figs 15P, R, S, U–W) with smooth median element and “comma-shaped” and lobate terminal elements (clearer in the posterior part, Figs 15S, V). Marginal zone rather wide anteriorly, relatively narrow posteriorly, both with vestibules, the posterior being very narrow; MPC straight and simple, about seven in the posterior zone, number anteriorly could not be determined; four slightly elongated adductor muscle scars, arranged in a curved vertical row, seen externally in the subcentral depression (Figs 15K, L)., Published as part of Danielopol, Dan L., Cabral, M. Cristina, Horne, David J., Namiotko, Tadeusz & Lord, Alan R., 2023, Reconciling diagnostic traits in living and fossil taxa: The taxonomy and evolution of the genus Microceratina (Crustacea, Ostracoda, Cytheruridae), pp. 301-340 in Zootaxa 5244 (4) on pages 328-330, DOI: 10.11646/zootaxa.5244.4.1, http://zenodo.org/record/7663518, {"references":["Brady, G. S. (1868) A monograph of the Recent British Ostracoda. Transactions of the Linnean Society of London, 26 (2), 353 - 495. https: // doi. org / 10.1111 / j. 1096.3642.1968. tb 00199","Brady, G. S. & Robertson, D. (1869) Dredging West of Ireland. Annals and Magazine of Natural History, Series 4, 3, 353 - 374. https: // doi. org / 10.1080 / 00222936908695961","Brady, G. S. & Norman, A. M. (1889) A monograph of the marine and freshwater Ostracoda of the North Atlantic and of North- Western Europe. Section I. Podocopa. Scientific Transactions of the Royal Dublin Society, Series II, IV, 63 - 270. https: // doi. org / 10.1080 / 00222938909460549","Sars, G. O. (1866) Oversigt af Norges Marine Ostracoder. Forhandlinger i Videnskabs-Selskabet i Christiania, 1865, 1 - 130.","Athersuch, J. & Horne, D. J. (1983) On Nannocythere pavo (Malcomson). A Stereo-Atlas of Ostracod Shells, 10, 29 - 38.","Malcomson, S. M. (1886) Recent Ostracoda of Belfast Lough. Report of Proceedings of the Belfast Natural Field Club, Appendix 9 (1884 - 1885), 259 - 264.","Athersuch, J. (1976) On Sagmatocythere napoliana (Puri). A Stereo-Atlas of Ostracod Shells, 3, 117 - 124.","Schafer, H. W. (1953) Uber Meeres- und Brackwasser-Ostracoden aus dem Deutschen K ¸ stengebiet. Mit 2: Mitteilung ¸ ber die Ostracodenfauna Griechenlands. Hydrobiologia, 5 (4), 351 - 389. https: // doi. org / 10.1007 / BF 00018667"]}