THE CHALK 53 



scutatus, which, with varieties of the same and allied 

 species, abounds in the Upper Chalk, and the more conical 

 Conulus conicus. The topmost zone, that of B. Macro- 

 nata, would yield a record of exuberant life, were the 

 chalk soft and horizontal. There was a rich development 

 of echinoderms (sea urchins and star fishes), but nothing 

 is perfect, owing to the hardness of the rock (Dr. Rowe). 

 The general difference in the life of the Chalk period is 

 the great development of Ammonites and other Cephalo- 

 pods in the Lower Chalk, and of sea urchins and other 

 echinoderms in the Upper, while the Middle Chalk is 

 wanting in the one and the other. Shark's teeth tell of 

 the larger inhabitants of the ocean that flowed above the 

 chalky bottom. 



Many quarries have been opened on the flanks of the 

 Chalk Downs, of which a large number are now disused. 

 They occur just where they are needed for chalk to lay 

 on the land, the pure chalk on the north of the Downs to 

 break up the heavy Tertiary clays, which largely cover 

 the north of the Island ; the more clayey beds of the Grey 

 Chalk on the south of the downs to stiffen the light loams 

 of the Greensand.* 



*Dr, A. W. Rowe. 



