. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 281. Fig. 134 Acrograptus? sp. a of Skevington, 1965. Llanfallteg Formation, latest Arenig, D. levigena Biozone, loc 52H, x 5, Q5172. Occurrence in south Wales. Fennian, Dionide levigena Biozone, Llanfallteg Formation; loc. 52W. Material. Q5172. Discussion. Skevington (1965) named his 'nov. sp. a' for Holm's 'mutation' of Didymograptus gracilis Tornquist. He noted that true gracilis had an asymmetrical proximal end, which we would now regard as sigmagraptine (Cooper & Fortey 1982: fig. 660- In 'sp. a' the or
. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 281. Fig. 134 Acrograptus? sp. a of Skevington, 1965. Llanfallteg Formation, latest Arenig, D. levigena Biozone, loc 52H, x 5, Q5172. Occurrence in south Wales. Fennian, Dionide levigena Biozone, Llanfallteg Formation; loc. 52W. Material. Q5172. Discussion. Skevington (1965) named his 'nov. sp. a' for Holm's 'mutation' of Didymograptus gracilis Tornquist. He noted that true gracilis had an asymmetrical proximal end, which we would now regard as sigmagraptine (Cooper & Fortey 1982: fig. 660- In 'sp. a' the origin of the thecae is almost symmetrical to either side of the sicula, the apertural lip of which projects as a minute tooth. From the Llanfallteg Formation—and from the same stratigraphical position as Skevington's examples—we have a gracile specimen with exactly the same proximal end. This is more complete than was the material from Oland, from which it differs only in having the sicula 01 mm longer, and the stipe width also being 01 mm wider. The sicula is 0-9 mm long and 0-25 mm wide at its base. The stipes are declined at about 140° initially but become more horizontal after th 2. Thecae have the very low inclination invariable in gracile species, 0 is a high acute angle and there appears to be a minute apertural denticle. Distance between thecal apertures varies between 0-8 and 10mm (equivalent to 10-12 thecae in 10mm). Stipe width at th 1 is 0-3 mm, increasing to no more than 0-4mm at th 17. Extremely thin species such as this are easy to overlook. However, the difference in proximal end structure from other Acrograptus spp. indicates that this sp. a is likely to prove a distinctive late Arenig form. Because of the difference from the typical sigmagraptine, it is included with question in Acrograptus. None of the Acrograptus species described by Boucek (1973) from Bohemia is as thin—the closest is apparently A. lipoldi, but this is reported to be 10 mm wide at
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