STRUCTURE OF THE VERTEBRATES 347 



roof of the skull. This group evolved in two major directions. 

 The smaller group evolved toward the mammal-like reptiles, and 

 eventually gave rise to the Class ^Mammalia. The other line 

 developed a second pair of fenestrae and form the reptilian sub- 

 class Diapsida. 



From the diapsid reptiles came the great reptilian groups of 

 the Age of Reptiles: (1) the dinosaurs which dominated the 

 earth for millions of years. Some were the largest land animals 

 known, some were marvellously armored, and others were bi- 

 pedal, swift, carnivorous and powerful. (2) The Pterosaurs were 

 carnivorous flying reptiles which became extinct with the de- 

 struction of the dinosaurs. Also included in this sub-class are 

 (3) Ichthyosaurs, ovo- viviparous animals highly specialized 

 for water life; (4) the living Sphenoclon of New Zealand,, sole 

 representative of a once large group; and (5) the Crocodilia, dis- 

 tributed throughout the tropical and semi-tropical regions of 

 the earth, which with their semi-aquatic habits survived in large 

 numbers. 



The birds, an offshoot of the diapsid stock, have left few fos- 

 sil remains. Archaeopteryx, the first known bird, lived in the 

 Jurassic. Toothed, water living birds have been found in the 

 Upper Jurassic and Cretaceous; but the immediate ancestors of 

 the birds are almost unknown. 



There are two problems which face the student of vertebrate 

 distribution in the transition from the ]Mesozoic to the Cenozoic 

 —the destruction of the great reptiles and the survival of the 

 mammals. The latter is more easily understood. Small, gener- 

 alized, active, slinking mammals could avoid the larger reptiles 

 during the many changes of the earth's surface; and their gen- 

 eralized structure would have adapted them for new environ- 

 ments. There are several theories for the practical extinction of 

 the larger reptiles. "Racial senescence" is used to express the 

 tremendous specialization of most of them, with their size, or 

 armor, or bony excrescences; but it is doubtful if a race does 

 become senile and die without other cause. Certain lizards to- 

 day, although smaller, have the same weird growths. Another 

 theory (Schuchert) accounts for the disappearance of the dino- 

 saurs as due to climatic changes. The late Cretaceous was a 

 period of elevation in America, at least, with the Rocky INIoun- 



