In the autumn of 1864, Mr. Ezekiel Williamson discovered fragments of trilobites in some slaty beds belonging to “Lower Lingula Flags” of the Geological Survey, on the right bank of the river Mawddach, a little above its junction with the Eden, and four and a half miles directly north from Dolgelly. Portions of an Olenus and of a Agnostus were found, but in too fragmentary a condition to be determined. Last summer Mr. J. Chamberlain Barlow, of Birmingham, found the same fossils in great abundance and good preservation on both sides of the Mawddach, opposite to Dolmelynllyn. During the present year I have, from these beds, added another species of Agnostus to the scanty fauna; and, after considerable trouble, in consequence of the rocks being greatly faulted, have Been able to determine the true position of this new fossiliferous zone.