twenty-seven inches; the width also is less, being 
ten inches across the metacarpals, while that of the 
Mastodon is twenty-two: but the one is a bird, the 
other a quadruped. ‘The toes are three in number, 
and present the same divisions with existing birds ; 
the inner toe having three, the middle four, the outer 
five phalanges. Some of the articulations of the toes 
of this noble specimen are remarkable for the manner 
in which they illustrate the mode of formation of the 
tracks. ‘These phalanges have become separated from 
the solid rock in which they were encased, so as to be 
removable at pleasure; and they thus show that the 
whole foot is not a simple impression in the rock 
which contains it, but a depression filled by foreign 
materials, i.e. by sand, clay, and other relics of 
pre-existing rocks. These materials had been gradually 
deposited in the mould formed by the bird’s foot, and 
are therefore independent of this rock, in the same 
way as the plaster-of-Paris cast of a tooth, or any 
other body, is independent of the mould to which it 
owes its form. The impressions are in gray sandstone. 
On the reversed surface of the slab is seen a small 
piece of broken quartz, about half an inch square. 
This piece forms a beautiful illustration of a part 
of the process by which the sandstone rocks are 
formed. 
