A new species of the placoderm fish Holonema is described from the Frasnian of Western Australia as H. westolli. A detailed account of the head and trunk shields is given; the palatoquadrate, opercular cartilage and gnathals are also described and receive special attention in view of the interest that attaches to these structures. Restorations of the armour are given in connexion with a discussion of the possible mode of life, and its convergent resemblance to antiarchs is noted. In a section on comparative morphology, some specimens of Arctolepis decipiens from Spitsbergen are partly redescribed; the variation in the dermal bones of the snout in Holonema westolli is analysed with respect to their possible ontogeny; and the structure of the nasal capsules and nasal openings in arthrodires is discussed, with the result that the usual elasmobranch model used in their interpretation is discarded; three possible interpretations of the snout in Holonema are proposed and one is tentatively accepted after a discussion of the available evidence, although it implies that some of the normal morphological relationships in the arthrodire snout have been seriously modified. The comparative morphology of the laterallines of the cheek and flank is considered, as well as of the jaw apparatus and gill cover. This last discussion is summarized in a general account of jaw suspension in placoderms. In a review of the family Holonematidae, the type and all referred genera (Holonema, Deirosteus, Rhenonema, Gyroplacosteus, Megaloplax, Deveonema) are tentatively defined, although the available information is not fully adequate for this purpose, and all named species and recent records are considered. The poorly known genera Aspidichthys, Glyptaspis, Groenlandaspis, Grazosteus, Tropidosteus and Arctonema, are excluded from the family. New material is described from Germany, Belgium and Scotland as Holonema spp. Throughout this review Holonema westolli is used as a standard of comparison rather than the poorly known type species H. rugosum. Finally the relationships and classification of the holonematids are considered. It is concluded that they are arthrodires at the advanced coccosteomorph level of organization, phylogenetically equally distinct from all other brachythoracids, and it is recommended that the Brachythoraci should be divided into the collateral taxa Holonematei and Eubrachythoracei, the former with the single family Holonematidae.