describe the forms of the various stones which compose the ribbed and pointed roof of a Gothic cathedral, the arched ridge or hump of the back requiring, in a somewhat similar way, a peculiar form and arrangement of plates. The apex of the ridge is covered by a strong hexagonal plate, fitted upon it like a cap or helmet, and which nearly corresponds in place to the flat central plate of the under side. There runs around it a border of variously formed plates, that diminish in size and increase in number towards the head, and which are separated, like the pieces of a dissected map, by deep sutures. They all present the tubercled surface. The eyes are placed in front, on a prominence considerably lower than the roof-like ridge of the back; the mouth seems to have opened, as in many fishes, in the edge of the creature's snout, where a line running along the back would bisect a line running along the belly; but this part is less perfectly shown by my specimens than any other. The two arms, or paddles, are placed so far forward as to give the body a disproportionate and decapitated appearance. From the shoulder to the elbow, if I may employ the terms, there is a swelling, muscular appearance, as in the human arm; the part below is flattened, so as to resemble the blade of an oar, and terminates in a strong, sharp point. The tail--the one leg on which, as exhibited in one of my specimens, the creature seems to stand--is of considerable length, more than equal to a third of the entire figure, and of an angular form, the base representing the part attached to the body, and the apex its termination. It was covered with small tubercled, rhomboidal plates, like scales, (see Plate I., fig. 3;) and where the internal structure is shown, there are appearances of a vertebral column, with rib-like processes standing out at a sharp angle. The ichthyolite, in my larger specimens, does not much exceed seven inches in length; and I despatched one to Agassiz, rather more than two years ago, whose extreme length did not exceed an inch. Such is a brief, and, I am afraid, imperfect sketch of a creature whose very type seems no longer to exist. But for the purposes of the geologist, the descriptions of the graver far exceed those of the pen, and the accompanying prints will serve to supply all that may be found wanting in the text. Fig. 1, in Plate I., and fig. 2, in Plate II., are both restorations--the first of the upper, and the second of the under, part of the creature. It may, however, encourage the confidence of the naturalist, who for the first time looks upon forms so strange, to be informed that Plate I., with its two figures, was submitted to Agassiz during his recent brief stay in Edinburgh, and that he as readily recognized in it the species of the two kinds which it exhibits, as he had previously recognized the species of the originals in the limestone. [Illustration: _PLATE II._]