. Bulletin of the British Museum (Natural History), Geology. . ij&^ L. *â â «*#» Fig. 68 Illaenopsis harrisoni (Postlethwaite & Goodchild 1886). Upper Arenig, Fennian, B. rushtoni Biozone, Pontyfenni Formation, loc. 23. a, b, well-preserved smaller cranidium, in relief, dorsal and oblique lateral views, x 6, ; c, latex cast of smaller pygidium, x 4, ; d, presumed transistory pygidium with indications of four segments, x 6, It. 19687; e, small cephalon in relief in nodule, showing eye on left (the apparent occipital ring is an artefact produced by the first thoracic se
. Bulletin of the British Museum (Natural History), Geology. . ij&^ L. *â â «*#» Fig. 68 Illaenopsis harrisoni (Postlethwaite & Goodchild 1886). Upper Arenig, Fennian, B. rushtoni Biozone, Pontyfenni Formation, loc. 23. a, b, well-preserved smaller cranidium, in relief, dorsal and oblique lateral views, x 6, ; c, latex cast of smaller pygidium, x 4, ; d, presumed transistory pygidium with indications of four segments, x 6, It. 19687; e, small cephalon in relief in nodule, showing eye on left (the apparent occipital ring is an artefact produced by the first thoracic segment), x 6, utmost fineness. The lines are bowed forwards over the glabella and pygidial axis. On the free cheeks they run almost parallel with the margin except where curving inwards in the ocular region; on the pygidial pleural fields they are more or less transverse. Discussion. This species is very close to /. thomsoni (Salter), as redescribed in Whittard (1961ft), particularly incorporating Procephalops hopense Whittard, 1967, into that species, as we sug- gested above. The lectotype (Whittard 19616: pi. 31, fig. 3) shows four clearly defined axial rings on the pygidium, which we cannot match on any specimen of /. harrisoni. We have observed the same feature on other specimens of /. thomsoni ( BGS GSM292) from the Hope Shales, and the well-defined rings seem to be associated with a small amount of transverse extension; in any case they are not visible on other specimens (Whittard 1961ft: pi. 31, fig. 4; 1966: pi. 50, fig. 7) which are little distorted. All the articulated specimens of /. thomsoni have wider thoracic pleurae, compared with /. harrisoni of similar size, such that the exoskeleton is more broadly oval, width only about two-thirds the length. Such wide thoracic pleurae can be matched on only the largest specimens of /. harrisoni. The holotype of Procephalops hopense is more elongate, but even here the axis is narrow, and there may have been
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