The principal evidence for Ordovician biogeog- raphy used in this account comes from trilobites and brachiopods, with important additional contributions from fossil Mollusca, especially nautiloids.Trilobites have been used for many years to discriminate biogeographic areas (e.g., Whittington and Hughes, 1972), and they are particularly useful as a highly speciose group with distinctive morphologies that tend towards endemicity which is related to, and dependent upon, past continental configurations and cli- matic zones. Brachiopods are sessile organisms