. 
2 ANIMALS OF THE PAST 
stone. And yet it is not essential for a speci- 
men to have had its animal matter replaced by 
some mineral in order that it may be classed as 
a fossil, for the Siberian Mammoths, found en- 
tombed in ice, are very properly spoken of as 
fossils, although the flesh of at least one of these 
animals was so fresh that it was eaten. Like- 
wise the mammoth tusks brought to market 
are termed fossil-ivory, although differing but 
little from the tusks of modern elephants. 
Many fossils indeed merit their popular ap- 
pellation of petrifactions, because they have 
been changed into stone by the slow removal 
of the animal or vegetable matter present and 
its replacement by some mineral, usually silica 
or some form of lime. But it is necessary to 
include ‘indications of plants or animals’ in 
the above definition because some of the best 
fossils may be merely impressions of plants or 
animals and no portion of the objects them- 
selves, and yet, as we shall see, some of our 
most important information has been gathered 
from these same imprints. 
Nearly all our knowledge of the plants that 
flourished in the past is based on the impres- 
