220 THE STOftY OF THE EARTH AND MAN. 



above a dozen large species from the Upper Meso- 

 zoic of England, and dates the first appearance of 

 the turtles in England about the time of the 

 Portland stone, or in the upper half of the Meso- 

 zoic ; but footprints supposed to be those of turtles 

 are found as far back as the Trias. Perhaps no 

 type of modern reptiles is more curiously special- 

 ized than these animals, yet we thus find them 

 contemporaneous with many generalized types, and 

 entering into existence perhaps as soon as they. 

 The turtles did not culminate in the Mesozoic, but 

 go on to be represented by more numerous and 

 larger species in the Tertiary and Modern. In the 

 case of the crocodiles, while they attained perhaps 

 a maximum toward the end of the Mesozoic, i f 

 was in a peculiar form. The crocodiles of this 

 old time had vertebrae with a hollow at each end 

 like the fishes, or with a projection in the front. 

 At the end of the Mesozoic this was changed, and 

 they assumed a better-knit back, with joints having 

 a ball behind and a socket in front. In the 

 Cretaceous age, species having these two kinds of 

 backbone were contemporaneous. Perhaps this im- 

 provement in the crocodilian back had something 

 to do with the persistence of this type after so 

 many others of the sea-lizards of the Mesozoic 

 had passed away. 



Of the fishes of the Mesozoic we need only say 

 that they were very abundant, and consisted of 

 sharks and ganoids of various types, until near 



