The trilobite Dikelokephalina is widespread across the Tremadocian Gondwana, Avalonia and Baltica palaeocontinents. The hitherto poorly known Anglo-Welsh species D. furca (Salter, 1866) is revised on the basis of new and well-preserved material. A new species, D. brenchleyi sp. nov., is described from the Anti-Atlas, Morocco. The two species together provide a fuller understanding of the genus, and of the family, Dikelokephalinidae (Kobayashi, 1934), to which it has been assigned. Suggestions that this family may be a junior synonym of Hungaiidae (Raymond, 1924) are considered premature. D. brenchleyi sp. nov. apparently shows variation in mature segment number, a most unusual feature in post-Cambrian trilobites. It is found in monospecific assemblages of large individuals which lack ventral median sutures, and a case is made that these individuals represent a mass gathering of individuals that have ceased to grow, for the purpose of fertilizing eggs, as happens in living Limulus. Copyright © 2010 John Wiley & Sons, Ltd.