rent 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.

To these last remarks there is one exception: the deep impression in
which the bird slipped in a lateral direction corresponds with an
elevation on the lower surface, in which the impression of these toes
is very distinctly disp