Article: The trilobite Encrinurus multisegmentatus (Portlock) and allied Middle and Upper Ordovician species

An Encrinurus multisegmentatus species-group is defined. The main diagnostic criterion is the arrangement of the tubercles on the cranidium; a system of notation is used to indicate the positions of the main glabellar tubercles. E. multisegmentatus (Portlock) from the Killey Bridge Beds, Tyrone, is redescribed, and the Middle and Upper Ordovician species which constitute the group are discussed. One new species, E. lamonti from the Lower Drummuck Group, Ayrshire, is established.