THE PROTOZOA OF THE SEA SHORE 



109 



water, in order that the motions of their pseudopods may be 

 seen. 



If we brush off fragments from the surface of a freshly broken 

 piece of chalk, and allow them to fall into a vessel of water, and 

 then examine the sediment under the microscope, we shall observe 

 that this sediment consists of minute shells, and fragments of 

 shells, of foraminifers. In fact, our chalk beds, as well as the 

 beds of certain limestones, consist mainly of vast deposits of the 



FIG. 61. SECTION OP A PIECE OF NUMMULITIC LIMESTONE 



shells of extinct foraminifera that at one time covered the floor of 

 the sea. Such deposits are still being formed, notably that which 

 now covers a vast area of the bed of the Atlantic Ocean at a 

 depth varying from about 300 to 3,000 fathoms. This deposit 

 consists mainly of the shells of a foraminifer called Globigerina 

 bulloides, a figure of which is given on the opposite page. 



The structure of chalk may be beautifully revealed by soaking 

 a small piece of the rock for some time in a solution of Canada 

 balsam, allowing it to become thoroughly dry, and then grinding 



