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 
Fic. 7.—Globigerina bulloides, 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 bulloides (Fig. 7), which, when the protozoan dies, 
sink to the bottom, that it 1s usually known as Globigerina ooze. 
Oy SERA) eee 
Fic. 8.—Globigerina 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 
