The fossils which form the subject of this paper possess considerable interest to the palæontologist for three reasons:— 1. They are the only forms of the kind yet announced from the Western Continent. 2. They are the first authentic fish-fossils yet found in the Silurian rocks of America, all previously-reported discoveries of a similar nature having proved erroneous in consequence of zoological or stratigraphical errors. 3. Some of them are the oldest fish and, consequently, the oldest vertebrates yet discovered in any part of the world, with the possible exception of Pander's “Conodonts,” which are, however, not yet generally received into the class of Fishes. The oldest fish-remains already known from this continent are those of the Corniferous Limestone of Ohio and of the Lower Devonian, near the mouth of the Gulf of St. Lawrence, in Canada. Palæontologically, the latter of these appears to be the earlier. The Corniferous Limestone of Ohio yields Macropetalichthys, Dinichthys, Onychodus , and allied forms, with a single specimen attributed to Coccosteus ; but the Canadian beds contain Coccosteus, Cephalaspis , and Ctenacanthus , or a form assigned to that genus. These, however, are all Devonian fossils, and need not detain us longer. No Silurian fish have yet been obtained in America. Galicia has yielded at least one species from rocks considered to be Upper Silurian, the nature of which justifies the age assigned to it. This was the specimen upon which Dr. Kner, in 1847, based his memoir