The trilobites Hamatolenus (Myopsolenus) douglasi sp. nov., Kerberodiscus succinctus gen. et sp. nov. and Serrodiscus ctenoa? Rushton, 1966 are described from the upper part of the Hell’s Mouth Grits, St Tudwal’s Peninsula, together with hexactinellid sponge spicules, trace fossils and a single inarticulate brachiopod. The fauna is of late lower Cambrian age, within the upper part of the protolenid-strenuellid Zone of the Comley Series of British nomenclature. From comparison with related faunas the horizon is correlated approximately with beds in New York State containing the Acimetopus faunule, the top of the Issafénien Stage of Morocco, and the uppermost lower Cambrian trilobite-bearing horizon in the Purley Shales of Warwickshire, but it is younger than the Pseudatops viola horizon in the Llanberis Slates. The top of the Hell’s Mouth Grits is lithologically equivalent to the uppermost Rhinog Grits of the Harlech Dome.

Smooth (effaced) eodiscid trilobites are shown to be polyphyletic, and to complement the evidence of the Hell’s Mouth eodiscids Runcinodiscus index gen. et sp. nov. is described from the lower Cambrian of Comley, Salop.

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