FOOTPRINTS OF UNKNOWN ANIMALS. 29 



itself from the dead l)ody, 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 diflerent 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 palseontological 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 



* Witli the exception of a single specimen of the bones of a foot, preserved 

 in the cabinet of Amherst College. — E. R. L. 



