The morphology of the trinucleid exoskeleton is reassessed and various new structures and terms defined. On the basis of new interpretations of cephalic morphology, particularly that of the fringe, of all known genera, a new subfamilial classification is proposed. Two new genera, Deanaspis and Whittardolithus, and the subfamily Reedolithinae are proposed, and the genus Botrioides Stetson resurrected. The existing subfamilies, Trinucleinae, Cryptolithinae and Marrolithinae are recast considerably and the Tretaspidinae, Novaspidinae and Incaiinae rejected. The Hanchungolithinae is left unmodified. Evolution within the family is discussed together with the distribution of genera in terms of recent palaeogeographic reconstructions. A review of the relations between phylogeny and ontogeny confirms the essentially recapitulative nature of trinucleid ontogenies. A list of current generic assignments of known species is given.