THE HALL OF FOSSIL VERTEBRATES 



much more nearly known. The estimates given on page 5 are 

 based on the very careful study of the subject made by C. D. 

 Walcott, the present Director of the U. S. Geological Survey. 

 In concluding his discussion Dr. Walcott stated his belief that 

 the duration of geological time (the entire period included in this 

 table) might be measured by tens of millions of years, but not 

 by single millions or by hundreds of millions." 



The most ancient of the extinct animals shown here are the 

 creatures of the Age of Reptiles, such as the Dinosaurs, or great 

 land reptiles, Mosasaurs, or great marine lizards, Ichthyosaurs, 

 or fish-lizards, and other smaller animals. These are millions of 

 years old. Some of the Dinosaurs are the largest known land 



RESTORATION OF TITANOTHERIUM, AN EXTINCT HOOFED MAMMAL OF WESTERN AMERICA 



The picture shows a bull, a cow and a calf 

 From the original watercolor, based on mounted skeleton and skulls in American Museum. 



animals, longer than the width of the exhibition hall, and so tall 

 that if they were standing on all fours their backs would reach 

 within a few feet of the ceiling. 



The greater part of the specimens are Mammals, or animals 



8 



