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.