FOSSIL ANIMALS 283 



called Dinosaurs, Ichthyosaurs, Pterodactyls, etc., of which 

 no living representatives are left. Some of these reptiles 

 had wings (Pterodactyls) and seem more like great birds 

 than true reptiles. In the bird class, too, there were, in the 

 same era, various enormous kinds now extinct, some of which 

 had teeth. 



An interesting example of the geologic succession of 

 related animals and one often referred to in books about 

 extinct animals, is that of the horse series. In lower Eocene 

 rocks is found an animal called Eohippus, about the size 

 of a fox, with four hoofed toes and the rudiment of a fifth 

 on forefeet and three hoofed toes on hind feet. This is the 

 first of a series of similar but always differing and ever- 

 more horse-like forms that are found in the rock strata suc- 

 cessively younger and higher, representing Miocene, Pliocene 

 and Quaternary periods. The hoofed toes disappear one 

 by one, the size of the whole animal is ever larger, and the 

 teeth are more and more like horse's teeth as we examine the 

 successively younger (more recent) members of the series, 

 until in the rocks of our present epoch we find the bones of 

 an animal which is essentially identical with the horse as we 

 know it today. 



Similar ancestral series have been discovered for the 

 deer, for certain pond snails, for the ammonites, for many 

 other kinds of animals, indeed. The first deer in the early 

 Miocene had no antlers. In the middle Miocene are found 

 small deer with small two-pronged antlers. In the upper 

 Miocene and lower Pliocene there are larger deer with 

 three-pronged, larger antlers. In the later Pliocene occur 

 four and five-pronged antlers, while in the Pleistocene are 

 remains of deer with branching antlers like those of the liv- 

 ing species. 



The fossil fishes of the earlier geologic periods are all 

 of the simpler, more primitive families of which none or 

 but few representatives occur today. Of the 12,000 known 



