. Bulletin of the British Museum (Natural History), Geology. 252 R. A. FORTEY & R. M. OWENS Tetragraptus (Tetragraptus) bigsbyi askerensis Monsen 1937 (Figs 107a, b) 1937 Tetragraptus bigsbyi var. askerensis Monsen: 172-173; pi. 4, figs 15, 16; pi. 5, fig. 35; pi. 13, figs 5, 6; pi. 19, figs 4, 6, 9. Holotype. Palaeontological Museum, Oslo, K 0679; from the Lower Didymograptus Shale, Phyllograptus angustifolius elongatus Biozone (late Arenig); Slemmestad, Norway. Diagnosis. See Monsen, 1937: 172. Occurrence in south Wales. Upper Arenig, Fennian, Bergamia rushtoni Biozone at loc. 24, Llwyn-
. Bulletin of the British Museum (Natural History), Geology. 252 R. A. FORTEY & R. M. OWENS Tetragraptus (Tetragraptus) bigsbyi askerensis Monsen 1937 (Figs 107a, b) 1937 Tetragraptus bigsbyi var. askerensis Monsen: 172-173; pi. 4, figs 15, 16; pi. 5, fig. 35; pi. 13, figs 5, 6; pi. 19, figs 4, 6, 9. Holotype. Palaeontological Museum, Oslo, K 0679; from the Lower Didymograptus Shale, Phyllograptus angustifolius elongatus Biozone (late Arenig); Slemmestad, Norway. Diagnosis. See Monsen, 1937: 172. Occurrence in south Wales. Upper Arenig, Fennian, Bergamia rushtoni Biozone at loc. 24, Llwyn-crwn. Material. Q5798; NMW Discussion. The lectotype of T. bigsbyi bigsbyi (Hall 1865) was figured by Skevington (1965), who discriminated this species from other reclined tetragraptids by its very steeply recurved stipes, producing a deeply U-shaped profile. The type is from a possible late mid-Arenig horizon at Levis, Quebec (Skevington 1965: 5). Elsewhere T. bigsbyi bigsbyi is recorded from the early Arenig (Cooper 1979; Cooper & Fortey 1982) to the uppermost Arenig (Skevington 1965). Monsen (1937) distinguished several 'varieties' which are available for subspecific names. The stipes of the type and other fully grown specimens of T. bigsbyi bigsbyi are at least 2-5 mm, and usually more than 3 mm, wide at th 7-8. Monsen (1937) described 'var.' askerensis from the late Arenig of Norway, with narrower stipes; there is no doubt that these include mature specimens, because her pi. 9, figs 4, 6, 9 are actually larger than most specimens of T. bigsbyi bigsbyi. Stipe width is not usually a good specific character, but it may vary at population level from one closely related taxon to another. Monsen's population includes at least twelve speci- mens. These may prove to be population variants of T. bigsbyi bigsbyi, or may represent a separate subspecific group. Because two specimens from south Wales also have stipes less than 2 mm wide, and are of the same age as Mons
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