The occurrence near St. David's, in South Wales, of rocks supposed to be of the age of the Tremadoc Slates of North Wales, was mentioned in a Report by the late Mr. Salter and myself to the British Association in 1866; and a list of the fossils which had up to that time been discovered in them was also given. Several new forms, however, have since been found in these rocks, and some of them very recently, during researches made at Ramsey Island by Messrs. Homfray, Lightbody, Hopkinson, and Kirshaw, in conjunction with myself. The Brachiopoda were figured by Mr. Davidson in his paper “On the Earliest British Brachiopoda,” in the Geological Magazine for July 1868; and a supposed land-plant was named by me Eophyton explanatum in the same publication for Dec. 1869; but with the exception of these, the whole of the fossils, comprising a rich and exceedingly interesting fauna, are as yet undescribed. In the present paper I propose to describe all these new forms, and also to give some account of the lithological characters of the strata in which they occur, their relation to the other formations, and their geographical distribution in the neighbourhood of St. David's. On the map (fig.1) it will be seen that there are three distinct patches of these rocks shown, viz.:—at Ramsey Island, on the eastern coast; at the north end of Whitesand Bay, and extending for some distance in a N.E. direction; and in a district about five miles east of