. Bulletin of the British Museum (Natural History). I I. Fig. 2 Cross-section through various brachial valves to illustrate the different types of bema. The dotted lines indicate the limits of the bema, a-d are entire and e-h are divided; c and d are elevated; g and h illustrate the more extreme modifications, a, Aegiria; b, Rurambonites; c, Leangella; d, Xenambonites; e, Bilobia; f, Anisopleurella; g, Eoplectodonla; h, Bimuria. Morocco), and Cocks (1970) revised the Silurian species of the superfamily. Many plectambonitacean species and genera have been erected or revised in other p
. Bulletin of the British Museum (Natural History). I I. Fig. 2 Cross-section through various brachial valves to illustrate the different types of bema. The dotted lines indicate the limits of the bema, a-d are entire and e-h are divided; c and d are elevated; g and h illustrate the more extreme modifications, a, Aegiria; b, Rurambonites; c, Leangella; d, Xenambonites; e, Bilobia; f, Anisopleurella; g, Eoplectodonla; h, Bimuria. Morocco), and Cocks (1970) revised the Silurian species of the superfamily. Many plectambonitacean species and genera have been erected or revised in other papers and monographs (see the references at the end of this work), but no revision of the group as a whole has been published since Williams (1965). MORPHOLOGICAL TERMINOLOGY In general we follow the nomenclature used by Williams {in Williams et al. 1965), but the following terms need further elucidation. Bema: An elevated pad of secondary shell material in the brachial valve originating from or close to the anterior end of the socket ridges or plates (Fig. 1). It may be divided into several types: a) entire ( Figs 2a-d), divided ( Figs 2e-h), or bilobed ( Fig. 1) b) elevated ( Figs 2c, d) or not elevated ( Fig. 2a) Cardinal process: The brachial valve attachment area for the diductor muscles. In the Plectambonitacea it is usually trifid (when viewed from the posterior), but may be a simple central ridge. The type of cardinal process may be divided into two main groups: a) simple, or not undercut, in which the central lobe is connected anteriorly to the median part of the valve floor (the notothyrial platform), often with a myophragm ( Fig- 3g) b) undercut, in which there is no anterior support for the central lobe ( Fig. 4a). 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 perfectly resemble the original Britis
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