Dissimilarities between lists of trilobite families and genera from particular areas of the world have been assessed using Simpson's index. Analysis of these indices by a non-metric multidimensional scaling technique has revealed groupings which are interpreted as faunal provinces. Four such provinces are present in the Lower Ordovician; in the Caradoc certain of these provinces have merged, and in the Late Ordovician a single fauna appears to be world wide.
Assuming that a faunal province originally extended over parts of a single continental mass, an assemblage of parts of present continents is proposed to give a palaeogeographical map which differs in certain respects from that of Smith, Briden, and Drewry. Relative movements between these continental blocks is thought to have removed barriers to migration and resulted in the merging of faunal provinces.