. Bulletin of the British Museum (Natural History), Geology. 12 R. A. FORTEY & R. M. OWENS ring. The use of the term baccula (-ae) for swellings at the base of the glabella follows Opik (1967) and Fortey (1975). We use the term axial shield (Henningsmoen 1957) for moulted exoskeletons consisting of cranidium + thorax and pygidium. Family AGNOSTIDAE Salter 1864 Genus LEIAGNOSTUS Jaekel 1909 Type species. Leiagnostus erraticus Jaekel 1909, by monotypy. Leiagnostus cf. erraticus Jaekel 1909 (Figs 14a, b) Material. Imperfect dorsal exoskeleton, BGS Pr557 + Pr581; pygidia: , NMW
. Bulletin of the British Museum (Natural History), Geology. 12 R. A. FORTEY & R. M. OWENS ring. The use of the term baccula (-ae) for swellings at the base of the glabella follows Opik (1967) and Fortey (1975). We use the term axial shield (Henningsmoen 1957) for moulted exoskeletons consisting of cranidium + thorax and pygidium. Family AGNOSTIDAE Salter 1864 Genus LEIAGNOSTUS Jaekel 1909 Type species. Leiagnostus erraticus Jaekel 1909, by monotypy. Leiagnostus cf. erraticus Jaekel 1909 (Figs 14a, b) Material. Imperfect dorsal exoskeleton, BGS Pr557 + Pr581; pygidia: , NMW Stratigraphical range. Upper Arenig, Fennian, zone of Bergamia rushtoni, Pontyfenni For- mation. Localities. Pontyfenni (loc. 23), and Cwmmiles (loc. 51). Discussion. So featureless an agnostid does not require description of the general morphology. One pygidium (Fig. 14b) shows details of the pygidial axial musculature. The axis appears to be subfusiform, with a terminal tubercle at about two-thirds pygidial length; the median tubercle is far forward and is flanked by two pairs of muscle impressions; two further pairs lie behind these. The articulating half ring is well shown on the large pygidium of Fig. 14a; it is short (sag.) with a narrow ring furrow, which is not deepened into pits at its lateral extremities. Using Opik's (1967: 71-72) categories of articulating devices, that of Leiagnostus appears to be of the basic (peronopsid) kind. The articulating device of Metagnostidae (Geragnostidae of authors, see Fortey 1980) is of the glyptagnostid kind: for example, Geragnostus (Tjernvik 1956: pi. 1, fig. 10), Arthrorhachis (Fortey 1980: pi. 2, fig. 15) and Galbagnostus (Whittington 1965: pi. 4, fig. 7). Hence Leiagnostus is not likely to be an effaced metagnostid if this criterion is of phylogenetic importance. Effacement itself is highly polyphyletic in the Agnostida, and Howell's (1935) family Leiagnostidae based on this aspect of morphology is clearly unsatisfactory. T
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