. Bulletin of the British Museum (Natural History), Geology. 362 CARADOCIAN BRACHIOPOD FAUNAS OF THE BALA DISTRICT Horizon and locality. Allt Ddu Group : all specimens from sandstone and siltstone exposures about 80 ft. north of the gutter on Craig y Gath (type locality of Howellites ultima Bancroft 1945 : 209). Discussion. Dolerorthis duftonensis (Reed) -prolixa subsp. nov. first occurs just above the Pont-y-Ceunant Ash and is common throughout the succeeding ashes and limestones of the Gelli-grin Group. A few specimens belonging to the genus have also been recovered from one exposure of the
. Bulletin of the British Museum (Natural History), Geology. 362 CARADOCIAN BRACHIOPOD FAUNAS OF THE BALA DISTRICT Horizon and locality. Allt Ddu Group : all specimens from sandstone and siltstone exposures about 80 ft. north of the gutter on Craig y Gath (type locality of Howellites ultima Bancroft 1945 : 209). Discussion. Dolerorthis duftonensis (Reed) -prolixa subsp. nov. first occurs just above the Pont-y-Ceunant Ash and is common throughout the succeeding ashes and limestones of the Gelli-grin Group. A few specimens belonging to the genus have also been recovered from one exposure of the highest Allt Ddu sandstones and siltstones and at first sight appear to be quite distinct from the new subspecies. In terms of shape and outline the differences are not important for the faintly sulcate brachial valve is slightly less than four-fifths as long as wide and over one- quarter as deep as long compared with a relative depth of about one-third for the pedicle valve. A fragment of what is probably the external mould of a brachial valve shows that the radial ornamentation was probably costate but the entire mould could not have been more than 8 mm. long and this condition could have been related to the small size of the valve. The moulds, however, are not the remains of young shells because the pallial sinus patterns are beautifully preserved for both valves. These impressions are usually found in only mature adult or gerontic specimens and they are not seen in the interiors of D. duftonensis prolixa until the shells are about twice the size of those from the Allt Ddu beds. In other words the specimens being described grew at about one-half the rate of their younger relatives but whether this difference reflects a genetic change or simply an unfavour- able environment prior to the deposition of the calcareous ashes, when many of the Gelli-grin brachiopod species tended to be larger than average, remains to be (a) (b) Fig. 7. The dorsal (a) and ventral (b) interna
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