The highly fossiliferous upper Darriwilian to lower Sandbian Elnes Formation of Norway presents an interesting insight into the general responses and preferences of Lower Palaeozoic faunas belonging to a stable, mud dominated middle to outer shelf environment in connection with a major transgressive to regressive system tract. The rich trilobite fauna consisting of nearly 100 taxa is closely linked to the changes in the environment, being most abundant in a muddy and siliciclastic dominated environment just above storm wave base. The fauna is highly endemic for the region and the remainder of Baltoscania. This monograph presents a taxonomic description of the total trilobite fauna, including a new genus and seven hitherto unknown species. New and extensive biostratigraphical data is presented on the trilobites together with a study on the biogeographical and ecological aspects of the faunas.