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1. Elevators 



2. Fossil Fishes 



FOURTH FLOOR 



Foreword on Fossil Vertebrates 



In the East Corridor, and the South Pavilion at the left, as well as in 

 the East Wing and Southeast Pavilion at the right are displayed fossil 

 mammals, reptiles and fishes. 



In a general way, fossils are the petrified remains of plants or animals 

 that lived at some past period of the earth's history. In many instances 

 we have not the objects themselves but only their casts or impressions in 

 the rocks. This is particularly the case with shells. Sometimes, as with 

 the bones of the great Irish elk the objects have been buried in swamps or 

 bogs, and in a few rare instances as with the mammoth and woolly rhinoce- 

 ros, entire animals have been preserved for thousands of years in ice or 

 frozen mud. Fossils are found in localities where the dead animals or 

 plants have gradually been buried under laj^ers of sediment to such a depth 

 that they come in contact with the mineral waters of the earth and finally 

 become petrified. Later through subsequent upheaval and erosion they 

 are again brought to or near the surface of the earth. Petrifaction is the 

 slow replacement of animal or vegetable material by such minerals as car- 

 bonate of lime or silica. The process is very slow and for this reason flesh 

 is never petrified. Fossil beds are found in every continent. In our own 

 countrj", Texas, Montana, Wyoming, and the I5ad Lands of South Dakota 

 are famous for their large fossil beds, and many of the finest and rarest 

 fossils in the Museum were obtained in these localities. 



As it takes thousands of 3- ears for the various layers of earth to accumu- 

 late over the bones, and for the latter to become petrified, the study of 

 fossils and the strata in which they are found is an important aid in de- 

 termining the age of the earth and the succession of life thereon. ]\[any 

 of the skeletons exhibited in these halls are of animals which lived from 

 30,000 to 20,000,000 years ago. To prepare a specimen for exhibition the 

 96 



