. Bulletin of the British Museum (Natural History). If M. G. LOCKLEY & A. WILLIAMS Horizons and localities. B 5918 from 'Llandeilo Flags' (probably Glyptograptus teretius- f!//!/.?Zone)ofWellfield, Builth; SM A. 104412 and A. 104451-2 from G. teretiiisciiliis sha\es exposed in the stream section east of Pen-Cerrig, 3 km north of Builth (SO 048537); SM A. 104446-8 and GSM 16547 from G. leretiiisculus shales exposed in the Trecoed stream section, 4 km north of Builth Wells (SO 054552); SM A. 104449 from G. leretiiisculus shales exposed in Dulas Brook. 55 km north of Builth Wells (SO 059566);
. Bulletin of the British Museum (Natural History). If M. G. LOCKLEY & A. WILLIAMS Horizons and localities. B 5918 from 'Llandeilo Flags' (probably Glyptograptus teretius- f!//!/.?Zone)ofWellfield, Builth; SM A. 104412 and A. 104451-2 from G. teretiiisciiliis sha\es exposed in the stream section east of Pen-Cerrig, 3 km north of Builth (SO 048537); SM A. 104446-8 and GSM 16547 from G. leretiiisculus shales exposed in the Trecoed stream section, 4 km north of Builth Wells (SO 054552); SM A. 104449 from G. leretiiisculus shales exposed in Dulas Brook. 55 km north of Builth Wells (SO 059566); GSM 16900-2 and 16918 from Llandeilo Shales of Hellpool, Wyeford, Builth; SM (2 specimens) from Nemagraplus gracilis shales exposed in Gwern-y-fed quarry, 15 km north of Builth (SO 030526). Discussion. Monobolina crassa sp. nov. differs from M. plumbea Salter in its greater biconvexity, well-developed posteromedian dorsal platform and relatively small ventral platform, and therefore must be regarded as specifically distinct from the older Shelve species. Although Davidson (1866:61) stated that 'in the Museum of the Geological Survey some specimens are labelled Hellpool, Wyeford Builth. This is "Upper Llandeilo" and it may be a mistake', it is now certain that Monobolina occurs sporadically in the Llandeilo successions of the Builth area ( G. leretiiisculus to N. gracilis Zones). This means that the genus, represented by the type species M. plumbea (Salter) in the Arenig rocks of the Shelve area and the related form described here, does not have such a restricted stratigraphical and geographical range as previously considered (Rowell in Williams et al. 1965;H270, Williams 1974: 39). dorsal umbo uscle tracks avote postero lateral scars. poslero-medion rim emboyments manlle canol impressions Fig. 42 Diagrammatic views of the interiors of the brachial (right) and pedicle (left) valves of Paienila fissiira [Addison ms] sp. nov., shou/ing th^ posteromedian (
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