14 



ZOOLOGY 



produces the great divergency of forms. In some cases the 

 protruding pseudopodia deposit a secondary shell, which 

 obliterates the outline of the primary shell, and usually masks 



Fig. 7. — Olobigerina buUoides, as seen in three positions. 



its form. The mud at the bottom of the Atlantic and other 

 seas is composed to such an extent of the calcareous shells 

 of Globigerina buUoides (Fig. 7), which, when the protozoan dies, 

 sink to the bottom, that it is usually known as Globigerina ooze. 



Fig. 8. — GloMgerina ooze from 1900 fathoms. 



The living Globigerina (Fig. 6) floats at the surface of the sea, the 

 protoplasm extending round the shell and forming a much vacuo- 

 lated envelope to it. Some slight idea of the enormous number 

 of these organisms which must have lived to build up the foram- 

 iniferous rocks which extend from the Palaeozoic times onward 

 may be formed from the fact that D'Orbigny estimated there 

 were 160,000 shells in a gramme of sand from the West Indies, 

 and Schultze gives 1,500,000 in 15 grammes of sand from 



