The project has involved the revision of the stratigraphy of the upper part of the Ordovician succession in central Portugal. Using evidence derived principally from the study of measured sections, with some limited geological mapping, this thesis first considers a new lithostratigraphy of the upper Ordovician deposits. The scheme presented includes four groups, twelve formations and eleven members. Most of the elements of the scheme are described here for the first time. The sedimentology of the deposits has been studied to allow an interpretation of the depositional environment. The history of sedimentation in the upper Ordovician is divided into three broad phases. In the first of these (phase A) storm-dominated deposition of detrital clastics took place on a wide, flat, shallow shelf. During the second phase (B) a localised area of igneous activity developed in the area of highest subsidence seen in the preceding phase. This igneous activity was accompanied by the deposition of some carbonate sediments. The third phase of deposition (C) is linked to the effects of the upper Ordovician glaciation. A review of most of the macrofauna recovered 1s given. The faunas of Portugal form a highly distinct and homogeneous "province" with those of Armorica, Spain, Sardinia. The palaeoecology of the faunas Montagne Noire and collected during this study are discussed. Thirteen broad associations (sensu Lockley 1983) are identified. Factors other than latitude-related influences play an important role in controlling the diversity, abundance and taxonomic constitution of the palaeocommunities. Twelve biozones are erected to cover the range of fossiliferous Ordovician strata examined in this project. The correlation between the biostratigraphie scheme erected herein and the international standard is discussed. An overview attempts to place the study area within the context of the peri-Gondwana shelf, and within the Variscan fold-belt.

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