It will readily be inferred from all this that the locality of Dura Den is entitled to much consideration in consequence of the variety of its interesting remains, not to speak of the diversity of views which the remains themselves have occasioned in so many quarters. The Pterichthys, Pamphractus, Homothorax, and Cephalaspis are all of the family _Lépidöides_, and have such a close affinity in outward form as readily, in mutilated specimens at least, to be mistaken for each other. The appendages of the head, having the appearance of wings, suggested the term _pterichthys_, the winged fish: the plates covering the body, according to their number and form, gave rise to the generic distinctions; and the species of each have subsequently been determined by minor differences. The external organs in all were enameled, and discover, like the fish of the period, the tuberculated surface. The Pterichthys of the more northern counties vary in size from nearly a foot to an inch in length, and generally the wings of these, so far as they have been figured in works, are extended horizontally and perpendicularly to the body. The Pamphractus of Dura Den are all nearly of a size—about two inches and a half in length,—the wings in every instance depressed and inclined to the sides, and in no instance of the twenty to thirty specimens exhumed from the rock, has the tail been appended, or a fragment of the caudal organ detected. The cephalaspis has only been found in the lower beds of the system, and highly important would be its discovery in the upper, where, however, we have reason to think the new genus Homothorax has been substituted in its place. Mrs. Dalgliesh of Dura, in whose collection we found a Glyptopomus, and a slab containing several impressions of the Pamphractus, has kindly, and with a commendable love of science, informed us that her quarries are freely open for the researches and explorations of geologists, and that every facility will be afforded them in their interesting task. In addition to the fossils already referred to, I find in the specimens of my collection returned from Neufchâtel, that two are labeled as Diplopterus, new species; two as Glypticus, new species; and one as Holoptychius, new species. This last is now figured in the “Monograph” as the Platygnathus Jamesoni. None of these are described in the narrative of the work, so that until his return from America, where palæontology will unquestionably reap much from his indomitable perseverance, his almost instinctive skill, and vast learning, we cannot expect that M. Agassiz will have leisure either to supplement the deficiencies of his great work, or confirm his former conclusions against the alterations suggested in his absence—suggested certainly in no small degree upon fanciful organization and mistaken assumption. [Illustration: Platygnathus Jamesoni.]