The most curious track, consisting of six digitated impressions, still remains. The first is seen on the left of the longer turtle-track, near the largest bird-track; the second is on the track; the third is above the track; the others cross the slab by fainter impressions. Each of them is composed by two feet, and each foot contains four toes, which are seen more distinctly in some impressions than in others. The largest of these double tracks is about three inches in diameter. Perhaps it would be useless to speculate upon what kind of animal they were made by. There is a similarity between these and the tracks of the Anomoepus Scambus, spoken of in the sixth group. In the latter, however, the toes are five and three. Some experienced persons think they are tracks of the mink, Mustela Lutreola, an animal common at the present day in these parts. This has five toes; but it may be in this as in some other digitigrades, that one of the toes in each foot does not make an impression; or perhaps it is safer to believe, till further investigation is made, that it was an animal of a construction not now existing. The direction of these tracks presents a puzzle we are not able to unravel; it exhibits the impressions of four toes, and we have supposed it might possess five. In either of these cases, we have no right to consider it a bird-track, but probably a reptile or a mammal. Admitting this to be the fact, we are unable to account for the direction of the steps, which is not alternate, as in the quadruped, but in straight lines. In other words, this animal, supposed to have four legs, gives us the impressions of two only, and both of these placed together. When the tridactylous tracks are attentively considered, compared with each other, and with the digitated tracks, they appear to exhibit the character of the impressions of the feet of birds so very decidedly, that it would require something more than a philosophic incredulity to question their ornithic origin. The other side of this slab contains interesting impressions. In the first place, this surface is covered with ripple-marks, each about two inches broad, extending with various degrees of distinctness across the slab, and having an interval of an inch. The width of the ridges is greater than in any of the specimens we have seen.