MESOZOIC ERA.- AGE OF REPTILES. 329 



No birds have been found. It may seem strange that 

 mammals should have been introduced before birds ; but 

 we find the explanation of this in the fact that birds are 

 a sub-branch of the reptilian branch of the vertebrate 

 stem. 



SECTION II. JURASSIC PERIOD. 



Name. These strata and the period they represent 

 are called Jurassic, because of their splendid develop- 

 ment in the folded structure of the Jura Mountains (Fig. 

 145, page 241) and their richness in fossils there. 



Rock-System. In England the Jurassic has been 

 subdivided into the Lias, the Oolite, and the Wealden ; 

 but we shall neglect these, and speak only of the whole 

 together. 



Coal. One point worthy of note here is the occurrence 

 of coal. The Jurassic coal-fields are far smaller than 

 those of the Carboniferous, but the mode of occurrence 

 of the coal is much the same. 



Examples of such coal are the Yorkshire and Brora 

 coal of Great Britain, and some of the coals of India and 

 China ; also the coals of eastern Virginia and North Caro- 

 lina. Of these last we shall speak again. Many Jurassic 

 coals are of excellent quality, though the average is 

 inferior to the coal of the Carboniferous. 



Plants. The characteristic families of the Jurassic 

 are Ferns, Conifers, and Cycads. Conifers and Cycads, 

 especially Cycads, culminated in this period ; they are 

 found in extreme abundance in connection with the 

 Jurassic coal in the form of leaves, trunks, and roots. 

 Some Jurassic plants and their living allies are shown in 

 Figs. 263-266. 



Animals. 



The culmination of the characteristic animals of the 

 especially reptiles, occurred in this middle 



