THE CRUST OF THE EARTH. 



176. BESIDES the larger class of fossil shells, there are numerous 

 infinitely more minute ones, which have largely contributed to 

 the formation of the actual crust of the globe, the principal of 

 which are the Foraminifera and Infusoria. 



The Foraminifera are marine animals of low organisation, 

 and generally of such extreme minuteness, that an ounce of 

 sea-sand will contain thr^e or four millions of them. The body 

 consists of uniform granules enclosed in a skin or membrane 



Fig. 97. Fig OS. Fig. 90. 



Serpula. 



Ammonites catena. 



Nautilus trun- 

 catus. 



having one or more cavities or digestive sacs. These animals 

 are regarded as polypes, and are protected by shells. Some of 

 these shells, such as the Orbiculina, contain a single cell. 

 Others, such as the Nodosaria, consist of several cells disposed 

 in conical or cylindrical directions. Other families have shells, 

 like that of the Nautilus, consisting of a succession of cells in 

 spiral forms. 



The Foraminifera derive their name from the structure of the 

 shell, which consists of one or more series of chambers separated 

 from one another by septa or partitions, in each of which there is 

 a small perforation called a foramen. 



Some specimens of these shells, on a highly magnified scale, are 

 given in figs. 100 to 106. 



These microscopic shells, of which from seven to eight hundred 

 fossil species have been discovered, are accumulated in enormous 

 numbers in the strata of the earth, and, in many cases, exclu- 

 sively form very considerable calcareous deposits, of which the 

 chalk and the cretaceous and tertiary strata present numerous 

 examples in all parts of the world. 



177. The Infusoria, the existing species of which are found in 

 130 



