2 ANIMALS OF THE PAST 



definition because some of the best fossils may be 

 merely impressions of plants or animals and no portion 

 of the objects themselves, 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 impressions of their leaves 

 left on the soft mud or smooth sand that later on hard- 

 ened into enduring stone. Such, too, are the trails 

 of creeping and crawling things, casts of the burrows of 

 worms and the many footprints of the reptiles, great 

 and small, that crept along the shore or stalked beside 

 the waters of the ancient seas. The creatures them- 

 selves have passed away, their massive bones even are 

 lost, but the prints of their feet are as plain to-day as 

 when they were first made. 



Many a crustacean, too, is known solely or mostly 

 by the cast of its shell, the hard parts having com- 

 pletely vanished, and the existence of birds in some 

 formations is revealed merely by the casts of their 

 eggs; and these natural casts must be included in the 

 category of fossils. 



Impressions of vertebrates may, indeed, be almost 

 as good as actual skeletons, as in the case of some fishes, 

 where the fine mud in which they were buried has be- 

 come changed to a rock, rivalling porcelain in texture; 

 the bones have either dissolved away or shattered into 

 dust at the splitting of the rock, but the imprint of 

 each little fin-ray and every thread-like bone is as 

 clearly defined as it would have been in a freshly pre- 

 pared skeleton. So fine, indeed, may have been the 

 mud, and so quiet for the time being the waters of the 

 ancient sea or lake, that not only have prints of bones 

 and leaves been found, but those of feathers and of the 

 skin of some reptiles, and even of such soft and delicate 



