The differences between the Caledonian orogenic belt in Britain and in Scandinavia strongly support terrane models for the evolution of the Caledonides in Ordovician to Silurian time. The ophiolites in Britain and Scandinavia are not fragments of the floor of a wide ocean. The provinciality of Cambrian and Ordovician faunas can be explained by their occurrence in allochthonous terranes. The persistent sedimentary facies belts that flank the continental margin of Laurentia contrast with belts that are discontinuous between Britain and Scandinavia. These features are better explained as consequences of continuing evolution of the margins of one ocean, Proto-Tethys, rather than by invoking a succession of oceans such as Iapetus.