MESOZOIC ERA: THE AGE OF REPTILES 523 



extinction of those species which, because of their lack of variability, 

 could not adapt themselves to the new conditions. 



Plan of Study. — For the sake of continuity in the study of the 

 various groups of animals and plants described, the life of the four 

 periods of the Mesozoic will not be studied separately, but the periods 

 in which the genera and species occur will often be referred to. 



Invertebrates 



Chalk. — Chalk is composed largely of the remains of Foraminifera. 

 Although these unicellular organisms have been found in Paleozoic 

 strata, being abundant in the Lower Carboniferous (Mississippian), 

 it was not until the Jurassic that they attained a great development. 

 Although conditions were very favorable for their increase in the 

 Jurassic of Europe (but not of America) and still more so in the Cre- 

 taceous, they were even more important as rock builders in the 

 Tertiary. 



The best known chalk deposits are those of which the cliffs of Dover, 

 England, and Dieppe, France, form a part; and because of their 

 conspicuous character, the name Cretaceous — Age of Chalk — was 

 given to the period in which they occur. In the United States also, 

 Cretaceous chalk is extensive. Chalk and chalky limestone many 

 hundred feet in thickness are found in the Lower Cretaceous series of 

 Texas; and another deposit in the Upper Cretaceous extends from 

 Texas northward through the Great Plains region, in Kansas, 

 Colorado, and Nebraska. However, this name is not altogether 

 appropriate, since by no means all the rocks of that period are composed 

 of chalk. It seems probable that the chalk of the Cretaceous was not 

 deposited in seas of great depth as is true- of the Globigerina (chalk) 

 ooze of to-day, which in portions of the ocean is being laid down at a 

 depth of 12,000 feet or more, but that the water was only moderately 

 deep. The occurrence in the chalk of certain mollusks which do not 

 seem to be of deep-sea species indicates this. Therefore, there is 

 little reason to believe that the great chalk beds of the Cretaceous were 

 deposited at depths of thousands of feet, and that the ocean bottom 

 was later raised to form dry land. An explanation for the purity of 

 the chalk, if deposited in comparatively shallow water, is to be found 

 in the conditions existing at the time. As a result of the low relief 

 of the land with its thick covering of vegetation, there was little 

 erosion, and the scanty sediments were laid down but a short distance 



