A slab from Turner's Falls, obtained for me by Dr. Deane in 1845, measuring two feet by two and a half, and two inches in thickness, contains at least ten different sets of impressions, varying from five inches in length to two and a half, with a proportionate length of stride from thirteen inches to six. All these are tridactylous, and represent at least four different species. In most of them the distinction of articulation is quite clear. The articulations of each toe can readily be counted, and they are found to agree with the general statement made above as to number. The impressions are singularly varied as to depth; some of them, perfectly distinct, are superficial, like those made by the fingers laid lightly on a mass of dough, while others are of sufficient depth nearly to bury the toes; some of the tracks cross each other, and, being of different sizes, belong to animals of different ages or different species. There is one curious instance of the tracks of a large and heavy bird, in which, from the softness of the mud, the bird slipped in a lateral direction, and then gained a firm footing; the mark of the first step, though deep, is ill-defined and uncertain; the space intervening between the tracks is superficially furrowed; in the settled step, which is the deepest, the toes are very strongly indicated. On the same surface are impressions of nails, which may have belonged to birds or chelonians. The inferior surface of the same slab exhibits appearances more superficial, less numerous, but generally regular. There are three sets of tracks entirely distinct from each other; two of them containing three tracks, and one containing two,--the latter being much the largest in size. In addition, there is one set of tracks, which are probably those of a tortoise. These marks present two other points quite observable and interesting. One is that they are displayed in relief, while those on the upper surface are in depression. The relief in this lower surface would be the cast of a cavity in the layer below; so the depressions in the upper surface would be moulds of casts above. The second point is the non-correspondence of the upper and lower surfaces; i.e. the depressions in the upper surface have not a general correspondence with the elevations on its inferior surface. The tracks above were made by different individuals and different species from those below. This leads to another interesting consideration, that in the thickness of this slab there must be a number of different layers, and in each of them there may be a different series of tracks.