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 JNIammoths, 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- 



