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 
