. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 271 Type material. In Nanjing Institute of Palaeontology. Late Arenig of south-west China, Didy- mograptus nexus Biozone (equivalent to 'Glyptograptus sinodentatus Biozone). Occurrence in south Wales. Bergamia rushtoni Biozone; loc. 21, Castell-y-waun; loc. 23, Pontyfenni; and Survey locality Carm. 37SE Wa4, Nant-yr-allwyn. Material. NMW , , , , ; Q5092-103, Q5182-5. Diagnosis. Subspecies of D. uniformis differing from the type in its closely spaced thecae, 14-16
. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 271 Type material. In Nanjing Institute of Palaeontology. Late Arenig of south-west China, Didy- mograptus nexus Biozone (equivalent to 'Glyptograptus sinodentatus Biozone). Occurrence in south Wales. Bergamia rushtoni Biozone; loc. 21, Castell-y-waun; loc. 23, Pontyfenni; and Survey locality Carm. 37SE Wa4, Nant-yr-allwyn. Material. NMW , , , , ; Q5092-103, Q5182-5. Diagnosis. Subspecies of D. uniformis differing from the type in its closely spaced thecae, 14-16 (?17) in 10mm in mature parts of stipe (th 10-15: 30 to 3-6mm). Proximal end declined, but proportions variable, involving 6-15 thecae to either side of sicula; distal stipes very gently declined or horizontal. Sicula 1-2-1-4mm long, development uncertain; acute angle between ventral walls of th l1 and th l2. Gentle increase in stipe width through first 20 thecae or so, distally uniform, but final stipe width varies between 1-20 mm and 16 mm on different speci- mens. On distal stipes 9 varies between 30° and 55°, (p between 45° and 60°. Discussion. The very close thecal spacing is exceptional in an extensiform, as noted by Ni (in Mu et al. 1979: 99). Ni also provided data on stipe width at various thecal numbers which is consistent with growth of our material (Fig. 125). Ni does not mention a stipe width greater than 1 mm, but her pi. 34, fig. 17 is 1-2 mm wide distally and hence within the range of variation of our material. The material figured by Ni has weakly declined proximal portions, projecting only about 2 mm above the distal stipes, like our specimen shown in Fig. 120e. However, the Welsh material does include specimens, such as NMW , with steeply declined proximal ends perhaps 4 mm above the distal stipes, and hence much more like the type of uniformis uniformis. The initial angle of declination varies between 100° and 130°, the lower angles being on th
Size: 3523px × 709px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorbritishmuseumnaturalhistory, bookcentury1900, bookcoll
