. Bulletin of the British Museum (Natural History), Geology. . d e f Fig. 3. Interareas of Eoplectodonta (a-e) and Plectodonta (f), pedicle valve above, brachial valve below, (a) E. duplicata (from BB 31738, Lower Llandovery, Gasworks Mudstone, Wales) with open delthyrium and chiUdial plates, behind them the central shaft and lateral processes of the cardinal process, (b) E. penkillensis (from BB 31840, Upper Llandovery, Hughley Shales, England) showing blade-like deltidial plates and chilidial plates. (c) E. transversalis (from BB 32422 Upper Llandovery, Lower Visby Marl, Sweden) showing chil
. Bulletin of the British Museum (Natural History), Geology. . d e f Fig. 3. Interareas of Eoplectodonta (a-e) and Plectodonta (f), pedicle valve above, brachial valve below, (a) E. duplicata (from BB 31738, Lower Llandovery, Gasworks Mudstone, Wales) with open delthyrium and chiUdial plates, behind them the central shaft and lateral processes of the cardinal process, (b) E. penkillensis (from BB 31840, Upper Llandovery, Hughley Shales, England) showing blade-like deltidial plates and chilidial plates. (c) E. transversalis (from BB 32422 Upper Llandovery, Lower Visby Marl, Sweden) showing chilidial plates and very small deltidial plates. (d) and (e) E. duvalii from the same locahty (both registered B 1587 from Lower Wenlock, Buildwas Beds, England) both with open delthyrium, but one specimen with entire chilidium and the other with discrete pair of chilidial plates, (f) Plectodonta mariae (from B 81377, Lower Devonian, Borschov Beds, ) showing deltidium and low but entire chilidium. What these diagrams do not convey is the relative angles of the interareas of the two valves, which are sometimes great, for example in E. duvalii ; however the interareas of P. mariae are set in nearly the same plane. been used in this paper for these structures. These clavicular plates grew in the same way as the socket plates of the Ordovician species with teeth, but the fact of their continued existence after the teeth had vanished from the stock, argues that the clavicular plates must have had some function in addition to their now obsolete role as socket plates. Havlicek, without discussion, describes the clavicular plates as brachiophores. However there is no direct evidence for this function, which is why an objective term has been coined here for the plates. It is possible that they might. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not pe
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