The third group is leptodactylous; foot usually small, but sometimes of medium size. Of it we have two specimens, viz.: ORNITHOPUS GALLINACEUS, and ORNITHOPUS GRACILIS. The former is so called from the resemblance to the domestic fowl: for convenience sake, in this and other instances, we use the whole for a part. It is about three inches in length, and the Ornithopus Gracilis about two. This latter specimen is particularly interesting. It consists of two parts, which open like the covers of a book. These covers present four impressions: first, the superficial, which is distinct, slender, and beautiful--the heel is broad; second, corresponding with this depression and on the inside, is a figure in relief as distinct as the depression; third, on the inside of the second cover is a depression corresponding with the relief last mentioned; fourth, on the outer side is a second relief corresponding with the second depression, but less distinct than either of the other three, still, however, exhibiting three toes pointing anteriorly, but the hind toe is wanting. The whole of this double slab forms a series of cameos and intaglios, measuring four inches by three, and in thickness an inch and a quarter. * * * * * GROUP FOURTH. Of the fourth group we have five specimens. The _Triaenopus_, so called from its resemblance to a trident, has besides three leptodactylous toes pointing forwards, a fourth extending backwards in a remarkable way, like the handle of a trident; the impression, however, being expanded so as to show an extensive displacement of the mud. All the specimens of Triaenopus are in a beautiful red shale, very thin and fragile, but presenting well-defined impressions, generally about three inches long. There are two species to this genus. Of the TRIAENOPUS EMMONSIANUS we notice three impressions in relief. In another specimen there is the appearance of a part of the toes of the Anomoepus Scambus, and on the upper side are seen two excavations corresponding with the three impressions. In the last slab, the track of the TRIAENOPUS BAILEYANUS appears to have been made by two feet placed successively in the same spot, which led President Hitchcock to suspect it might have been made by a quadruped. One of the specimens has the Triaenopus tracks intermixed in a peculiar way with other impressions. The specimen representing the genus HARPEDACTYLUS is larger than the preceding; and, though leptodactylous, the toes are much broader and also more curved, whence the name Harpedactylus, _sickle-finger_, from _harpe_ and _daktylos_. * * * * * GROUP FIFTH.