. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 265 Begin's Hill Quarry. O. M. B. Bulman recollected at this site; in an unpublished manuscript I have seen he records the species also from the underlying Phyllograptus typus Biozone. Both these occurrences belong within the Middle Arenig, probably in the Chewtonian-Castlemainian 1 interval of the Australian stratigraphic standard. Other material. Four other specimens in Hall's type series, GSC 914. Diagnosis. Didymograptus (Expansograptus) with stipes declined proximally (in range 135°- 155°) and distal stipes
. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 265 Begin's Hill Quarry. O. M. B. Bulman recollected at this site; in an unpublished manuscript I have seen he records the species also from the underlying Phyllograptus typus Biozone. Both these occurrences belong within the Middle Arenig, probably in the Chewtonian-Castlemainian 1 interval of the Australian stratigraphic standard. Other material. Four other specimens in Hall's type series, GSC 914. Diagnosis. Didymograptus (Expansograptus) with stipes declined proximally (in range 135°- 155°) and distal stipes horizontal to very slightly declined. Isograptid development; ventral walls of th ll and th l2 include a high angle; sicula l-3-l-5mm long, stipe width at th 1 0-7-0-8 mm. Stipe expansion rather steady until about th 20 and very little thereafter, to maximum between 1-5 and 1-75 mm. Distal thecae moderately flared at apertures and with tiny denticles, but interthecal septum inclined at 20°-35° to dorsal wall; distal thecal spacing 11-12 in 10mm (th 10-15 distance in range 3-8-4-2mm); always less than 90° and usually about 60°, so that apertures cut back about one-third of total stipe width distally. Discussion. Hall described the general features of this species, and the essential characters in the diagnosis require no further comment. Most of the original periderm has disappeared from the types. Although not present in south Wales, the species is present in north Wales associated with a Whitlandian trilobite fauna, and in the Lake District. Hall's types are a small popu- lation, but there are enough examples to suggest that previous views of D. nitidus have been too inclusive. A shallow-deflexed shape, with declined proximal end and more horizontal distal stipes, is regarded as a necessary character of the species, because such stipe characters seem to be fairly constant in other Expansograptus spp. Hence this would exclude such supposed nitidus as that in Berry (1960:
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