The type sections of the Ordovician, as defined by Charles Lapworth, are in Wales and Shropshire. The system was divided by Lapworth into three series: the Arenig, the Llandeilo, and the Bala. These three series are found nearly complete near Shelve in Shropshire. There has been considerable confusion in the use of the terms Llandeilo and Bala, largely owing to attempted correlation with beds of similar age in the Lake District. Professor Sedgwick referred all the formations above the Arenig to the Bala group; but according to Professor Marr the Llandeilo series of Murchison was subsequently used in the sense of the lower Bala of Sedgwick; the middle Bala was regarded as the Caradoc series; and Ash-gillian has been proposed for the upper Bala. It is rather a curious circumstance that the Bala, under the above classification, becomes more nearly the equivalent of Ulrich’s Ordovician, since Ulrich . . .