During a few days' holiday in South Pembrokeshire last summer, I went to the neighbourhood of St. David's. I wished to see the Cambrian and Lower Silurian beds recently mapped by Mr. T. Aveline, and to learn if they presented the same characters as the corresponding beds in North Wales. On this point I hope to have an opportunity of laying some particulars before the Society. My object now is only to point out the locality and geological place of a gigantic Trilobite long looked for in Britain, and lately, I must say accidentally, found by me. I believed I was working at Solva Harbour, in Llandeilo flags*, but by good fortune I had landed instead in a parallel creek a mile to the westward, at the junction of the red and purple Cambrian grits with the Lingula-slates. Porth-rhaw is a small boat-creek, a mile S. of Whitchurch, on the St. David's road. Black slates occur on both sides of it; but on the west side they pass into and rest upon the red and purple Cambrians; on the east they form magnificent cliffs of vertical or highly curved slates and flags. Sometimes these cliffs show sheets of rippled rocks 150 feet to the water's edge. Though much curved, the general dip is to the east and south-east; but I have not traced the slates further in this direction than the Cradle Rock; nor could I find fossils except at one point, where they are in hundreds. This was at