On one of my visits to the Silurian district of the Pentland Hills, in the beginning of last summer, I was fortunate enough to discover several fragments of a large crustacean, which I submitted to the inspection of Mr Page, who pronounced them to be undoubted fragments of Slimonia acuminata; they consist of a telson or tail-plate, 4½ inches in length by nearly two inches in breadth, showing the seriatures running down both sides, from the top to very near the point; also the anal plate of probably the same individual, along with some fragments of body rings, showing the sculpturings. The genus Slimonia, as most of you will be aware, belongs to the family Eurypteridae, and was detached from Pterygotus and erected into a new genus by our president, Mr Page. It differs from Pterygotus by having no prehensile claws; also in the shape of the carapace, and other well-known characters. The first notice we have of this large and interesting crustacean occurring in these beds is by Messrs Salter and Geikie, in the ‶Memoir of the Geological Survey Sheet, No. 32,″ where they describe a fragment found by their collector, Mr Gibbs, as the serrate base of one of the great swimming feet. This, they say, is all that occurred to their collector’s diligent search, but is invaluable as connecting the beds with those of Lesmahagow. One very remarkable circumstance connected with this specimen of theirs, is the finding of it in a bed associated with abundance of