770 



GEOLOGY 



tributions to the chalk of the period, and they were concerned in 

 the formation of the greensand, scarcely less characteristic of the 

 period than the chalk. While some of these minute organisms live 

 on shallow bottoms, on fixed algae, and in abysmal water, they are 

 chiefly inhabitants of the surface waters of the open sea. 



Sea-urchins were quite abundant, and lent one of its character- 



Fig. 523. Cretaceous Cephalopods: a, Nautilus meekanus Whitf., ono of the 

 simplest types of closely coiled cephalopoda; b, Helicoccras stephensoni 

 Whitf., an ammonite coiled in a heliciform spiral, and c, its highly coin- 

 plicated suture; d, Prionotropis woolgari (Mantell), a normal ammonite 

 with ornamented shell and e, complex sutures; /, Ptychoceras era 

 Whitf., an ammonite shell which is recurved upon itself, but not coil<-<l; 

 gr, suture of/; h, Scaphites nodosus Owen, an ammonite showing a slight 

 tendency to uncoil in the last volution; i, Baculitcs grand is M. anl II. 



