. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 207 in ej and e2, and it is this feature more than any other that distinguishes them from B. rushtoni, in which auxiliary pits are but rarely developed (see above). It is likely that B. rushtoni has evolved from B. inquilina, B. matura or a similar form by the total or almost total loss of auxiliary pits, and a similar reduction has taken place in B. rhodesi. B. inquilina and B. matura are of late Whitlandian or earliest Fennian age, and thus occupy an appropriate stratigraphical position to have given rise to B
. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 207 in ej and e2, and it is this feature more than any other that distinguishes them from B. rushtoni, in which auxiliary pits are but rarely developed (see above). It is likely that B. rushtoni has evolved from B. inquilina, B. matura or a similar form by the total or almost total loss of auxiliary pits, and a similar reduction has taken place in B. rhodesi. B. inquilina and B. matura are of late Whitlandian or earliest Fennian age, and thus occupy an appropriate stratigraphical position to have given rise to B. rushtoni in the Pontyfenni Formation and B. rhodesi in the Tankerville Flags. A further stage in the simplification of the fringe is seen in the Llanvirn species B. artemis Rushton & Hughes (1981: 630; pi. 2, figs 1-14, 16, 17) from the Great Paxton Borehole; here E: and E2 (with 12-15 pits per half-arc) converge posterolateral^ to form twin pits at about RIO, and become a single arc in the last three radii. In is the only inner arc developed. The dorsal exoskeleton of B. rushtoni shows a striking similarity to Anebolithus simplicior (Whittard) (Hughes et al. 1975: pi. 1, figs 17, 19), particularly in its cephalic sculpture and in the presence of deep radial sulci on the fringe. Without a lower lamella it might be easy to confuse the two, but they can be readily distinguished by the spacing of the sulci in front of the glabella. The interradial areas are narrower than the sulci in A. simplicior (Hughes et al. 1975: pi. 1, fig. 17), but much wider in B. rushtoni ( Fig. 72e). Bergamia sp. A (Fig. 73) Material. Incomplete external mould of cranidium, NMW , from Whitlandian, Blaen- cediw Formation, Blaencediw Quarry (loc. 29). Description. The pseudofrontal lobe appears to be rounded and somewhat swollen; traces of a reticulate sculpture can be seen on the genal lobes. A half-arc has 24 radii in deep, rather narrow radial sulci; three pits can be seen in one of the
Size: 2392px × 1045px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorbritishmuseumnaturalhistory, bookcentury1900, bookcoll
