. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 169 holotype because it differs from that of any other cyclopygid. The cranidium is wider than any Microparia species but does not differ in structure. Maximum cranidial width is attained some distance in front of the posterior cranidial margin, this being one and a half times the sagittal length, or slightly more. The specimen from the Pontyfenni is preserved in relief, showing very gentle transverse convexity, but a steep downward slope around the edge of the forward cranidial margin. Palpebral rims are compar
. Bulletin of the British Museum (Natural History), Geology. ARENIG IN SOUTH WALES 169 holotype because it differs from that of any other cyclopygid. The cranidium is wider than any Microparia species but does not differ in structure. Maximum cranidial width is attained some distance in front of the posterior cranidial margin, this being one and a half times the sagittal length, or slightly more. The specimen from the Pontyfenni is preserved in relief, showing very gentle transverse convexity, but a steep downward slope around the edge of the forward cranidial margin. Palpebral rims are comparatively well defined anteriorly, where they are narrow and tucked beneath the frontal part of the median cephalic lobe; backwards they are wider but become fainter, being hardly discernable behind the point of maximum cranidial width. However, the pleuroccipital furrows are faintly indicated at the posterior cranidial margin, indicating that the glabella there occupies about three-fifths the maximum cranidial width. Only the first three thoracic segments are preserved, but we have based the reconstruction (Fig. 47) on the assumption that there were five segments as in all Microparia. The axis is very wide; on the anterior segment the pleurae are only about one-third the axial width (tr.). Otherwise the thoracic structure is as conservative as in other cyclopygids. The first thoracic ring shows well the characteristic sagittal elongation of the first articulating half ring. The transverse pygidium has a gently arcuate posterior outline, and wide, weak border only developed laterally. There is no trace of the axis, but some indication of its extent is given by the inward extension of the narrow (exsag.) facets, which show that it occupied about half the pygidial width. Doublure wide, occupying more than a third of the sagittal length, and slightly wider laterally, carrying 7-8 terrace lines. Discussion. The cranidium from the Pontyfenni Formation is so like that from the Llanfall
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