This Bulletin is the result of a palaeozoological and stratigraphic study of Australian species of the trilobite family Dolichometopidae, which constitutes a substantial part of the Middle Cambrian fauna. The main result in stratigraphy is the discovery of a temporal sequence of some nine assemblages of taxa of dolichometopids constituting a scale of biostratigraphic operational units equal in value to zones and operational in geological dating of strata and faunas; and in palaeozoology-morphogenesis, morphological observations regarding the structure of the cephalon and the hitherto unsuspected diversity of the Australian dolichometopids. The material from Queensland has been studied exhaustively; the dolichometopids from the Northern Territory, however, especially from Huckitta and Lucy Creek area, are in need of purposeful collecting; the same applies to central Australia and New South Wales. Of the described forms Deiradonyx toddi comes from a collection made by the Amadeus Basin Geological Party; Fuchouia" labda was collected by Mr J. N. Casey; Fuchouia sp. novo aff. atopa by Mr M. A. Randal, and Parapoliella sp. novo by the two last named -all of this Bureau; the rest of the 1l1ateI"ial has been collected by the present author in the period 1948-1962; the whole material belongs to the Commonwealth Palaeontological Collection, whose type numbers are mar.ked by the prefix. CPC.