FOOTPRINTS OF UNKNOWN ANIMALS, 29 
itself from the dead body, which floats on the water. Whilst 
the body is driven away and dissolved by the water, the 
lower jawbone falls down to the bottom of the water and is 
there enclosed in the mud. This explains the remark- 
able fact that in a stratum of limestone of the Jurassic 
system near Oxford, in the slates of Stonesfield, as yet only 
the lower jawbones of numerous pouched animals (Mar- 
supials) have been found. They are the most ancient 
mammals known, and of the whole of the rest of their bodies 
not a single bone exists. The opponents of the theory of 
development, according to their usual logic, would from this 
fact be obliged to draw the conclusion that the lower jaw- 
bone was the only bone in the body of those animals. 
Footprints are very instructive when we attempt to 
estimate the many accidents which so arbitrarily influence 
our knowledge of fossils; they are found in great numbers 
in different extensive layers of sandstone ; for example, in 
the red sandstone of Connecticut, in North America. These 
footprints ‘were evidently made by vertebrate animals, 
probably by reptiles, of whose bodies not the slightest trace 
has been preserved.* The impressions which their feet 
have left on the mud alone betray the former existence of 
these otherwise unknown animals. 
The accidents which, besides these, determine the limits 
of our palzontological knowledge, may be inferred from 
the fact that we know of only one or two specimens of very 
many important petrifactions. It is not ten years since we 
became acquainted with the imperfect impression of a bird 
in the Jurassic or Oolitic system, the knowledge of which 
* With the exception of asingle specimen of the bones of a foot, preserved 
in the cabinet of Amherst College.—E. R. L. 
