6 THE COLLECTION OF FOSSIL VERTEBRATES 
tions of man are not found, and the remains of lower ani- 
mals which they contain are unlike any now living—the more 
unlike as the rock is more ancient. These remains are called 
josstls. They consist only of the hard parts of animals (bones, 
shells, spines etc.). The soft parts are never preserved, and 
only very rarely is some trace of skin or hair, horns or hoofs, to 
be distinguished. As in the course of ages the mud or sand in 
which they are buried changes to rock, so little by little the 
fossils have been changed by heat, pressure and especially by 
the slow infiltration of mineralized waters into brittle, stony 
material, while retaining their outward form and usually their 
peculiar structure. But mud and clay, in changing into rock, 
settle down and contract considerably, and the fossils are flat- 
tened out correspondingly, sometimes to such a degree, in the 
case of a rock which has once been a soft, oozy mud, that they 
suggest rather a picture or a bas-relief than the original form 
of the animal. The fossil skeletons of marine reptiles and fishes 
on the walls of the corridor hall and in the case opposite the 
elevator have been flattened out in this manner, especially the 
Ichthyosaur skeletons. 
From fossils we can interpret the history of the world of life 
during the long ages before man appeared. The science which 
Science of deals with the ancient history and evolution of the 
Paleon- animal kingdom is Paleontology (zadaios, ancient, 
tology. ovra, living beings, -Aoyéa, science). It tells us of a 
long period of time before Man appeared, probably millions of 
years, during which Mammals of great size and unfamiliar form 
were the dominant animals—of a yet longer era before that, 
during which huge Reptiles were rulers of earth, sea and air— 
and of other still more ancient periods during which Amphibians, 
Fish and Invertebrate Animals held sway in turn. Vertebrate 
Paleontology deals only with the higher classes of fossil animals, 
the Vertebrata, or those that have backbones (fish, amphibians, 
reptiles, birds and mammals). 
Earth-history or geological time has been divided into many 
Geological Parts according to the evidence furnished by the rocks 
Time. and the fossils contained therein. The principal sub- 
divisions are shown in the accompanying table: 
