Eoplectodonta (Ygerodiscus) Havliček, 1967

Musteikis, Petras & Cocks, L. Robin M., 2004, Strophomenide and orthotetide Silurian brachiopods from the Baltic region, with particular reference to Lithuanian boreholes, Acta Palaeontologica Polonica 49 (3), pp. 455-482 : 472

Subgenus Eoplectodonta (Ygerodiscus) Havliček, 1967

Remarks.—The type species of the subgenus is E. (Y.) undulata (Salter in Phillips and Salter, 1848), from the Middle Llandovery of Wales, revised by Cocks (1970). There is disagreement whether Ygerodiscus should be recognised as a separate taxon. Temple (1987) put E. (Y.) undulata within the synonymy of E. (E.) duplicata , the type species of Eoplectodonta (Eoplectodonta) ; however, whole populations of plectambonitoids appear to have the undulose shell so typical of Ygerodiscus, and so we retain the name as a subgenus here within Eoplectodonta so as to be able to distinguish such specimens. From Baltica E. (Y.) undulata has been recorded (as Sowerbyella undulata ) from the Middle Llandovery of the Choldre borehole, Latvia, by Rybnikova (1967: 187), but it is doubtful from her illustrations that the specimens are actually E. (Ygerodiscus) and are more probably E. ( Eoplectodonta ) and possibly E. (E.) penkillensis . Rybnikova (1967: 191) also described the species Plectodonta propinqua (from the Lower Llandovery of the same borehole), which from her illustrations should be attributed to E. (Ygerodiscus), but we consider that E. (Y.) propinqua is probably a junior synonym of E. (Y.) undulata ( Fig. 8J–L). From Lithuania we have recognised undulate Eoplectodonta from the lower Wenlock upwards, with the quantity becoming greater in the middle and upper Wenlock, this species is described below as Eoplectodonta (Ygerodiscus) bella sp. nov. Rare shells in the Riga Formation (lower Homerian) can be identified as E. (Y.) cornuta ( Davidson, 1883) , whose distribution is also listed in the Appendix.