The Silurian, Devonian, and Lower Carboniferous rocks of the Mississippi basin of the United States consist of thin limestones, and form altogether an insignificant mass when contrasted with the enormously thick deposits of the same age on the Atlantic border in Pennsylvania and the adjoining States, where the beds of corresponding age, composed for the most part of sandstones and shales, reach a thickness of from twenty to thirty thousand feet. Of these western limestones the Niagara group, the equivalent of the English Wenlock beds, has yielded a very rich harvest of fossil remains in almost all places where its beds are well exposed; but, being a typical dolomite, its fossils almost always occur as casts, the shells having been, except in very rare cases, removed by the percolation of acidulated water. Among the fossils of the Niagara Limestone is a great abundance of fragments of various kinds of Polyzoa, especially of the family of Fenestellids, but in so broken and mutilated a condition that reference to species is impossible. Indeed there is room for believing that of the descriptions already published several may, in some cases, be founded on the same species. During a residence of some years on the Niagara Limestone, at Yellow Springs, Green Co., Ohio, I obtained, among other Polyzoa, most of which were t~o imperfect for description, numerous specimens of a large and striking Fenestellid, quite distinct from every thing in the family already described, and yet showing features that fill an existing gap,and