The term May Hill, proposed by Sedgwick, is sometimes used as synonymous with Llandovery. This classification omits a somewhat important set of beds intercalated between those of the Llandovery and Wenlock series known as the Tarannon shales, and in Britain if we were to classify afresh, it would be more convenient to include some of the beds formerly referred to the Ludlow in the Wenlock. I shall, however, adopt the old and well-established classification, adding the term Tarannon to Llandovery, and speaking of the Llandovery-Tarannon series. The nature of the two classifications is shown in the following table: Old New Palæontological Stages. Classification. Classification. Classification. 1 Upper Ludlow } } } 2 Aymestry Limestone } Ludlow } Downtonian } 3 Lower Ludlow } } Fauna } } with 4 Wenlock Limestone } } } _Encrinurus_ 5 Wenlock Shale } Wenlock } Salopian } 6 Woolhope Limestone } } } 7 Tarannon Shales } } Fauna 8 Upper Llandovery } Llandovery } Valentian } with 9 Lower Llandovery } } } _Harpes_ [Illustration: Fig. 18. L = Ludlow. W = Wenlock. Ll-T = Llandovery-Tarannon.] _Description of the strata._ Lithologically the Silurian deposits of Britain form a continuation of those of the Ordovician period, with a local interruption due to the elevation of portions of Wales and the Welsh borders at the close of Ordovician times. Elsewhere we find a predominance of shales passing into grits at the top of the system, the change indicating the incoming of the shallow-water phase before the commencement of the second continental period. Particular stress is laid upon the predominant shaley character of the beds, for, on account of the richness and variety of the faunas of the calcareous rocks, greater attention is naturally paid to them in geological works, and the student may get a false idea of their relative importance. An attempt is made below (Fig. 18) to give a general idea of the variations in lithological characters of the Silurian rocks in different parts of Britain. The Silurian strata are mostly found in the same localities as those which furnish exposures of the rocks of Ordovician age. The development in the typical Silurian region of the Welsh borders is characterised by the abundance of calcareous matter which is found there as compared with that which exists in the other British localities.