KHIZOPODA. 



33 



especially of the shells of Glohigerina (fig. 10). The name of 

 "Globigerina mud" is often given to this deep-sea ooze, and it is 

 remarkable as having a close resemblance to the well-known White 

 Chalk. Thus, if we examine chalk under the microscope, we find 

 that it is also comjiosed to a large extent of the shells of Foram- 

 inifera, and especially of the shells of (Jlohigerina (fig. 11). We 

 may therefore regard the " Glohigerina ooze " of the deep Atlantic 

 and Pacific as a modern representative of the great geological 

 formation known as the White Chalk — a formation which may be 

 raany hundreds of feet in thickness, but is nevertheless principally 

 made up of the accumulated shells of these minute organisms. 

 Many of the massive limestones of the earth's crust are, in like 

 manner, largely made up of the shells of Fnrami/nifera. A striking 

 example of this is found in the so-called " Nummulitic Limestone," 

 which owes its name to the fact tliat it is largely made up of the 

 coin-shaped shells of a large Foraminifei-, termed the Nummulite. 

 Nummulites (fig. 12) vary in size fi-oni the size of a split pea up to 

 an inch in diameter, and they are now almost wholly extinct. 



Fig. 12. — NuramulUcs hruigatus. 



The Nummulitic limestone stretches from France on the west to the frontier* 

 of China on the east, and is almost everywliere readily recognisable as a dis- 

 tinct formation. It attains in places a thickness of several thousand feet, and 

 is especially largely developed in the Alps. It has an liistorie interest from 

 the fact that the Pyramids are huilt of it, ami that the Nummulites in it were 

 noticed by Herodotus, " the father of history." 



Order IV. Radiolaria. 



The third order of the Rhizopoda is that of the Radiolaria, essen- 

 tially distinguished by the fact that the sarcode-body has generally 

 the power of secreting a "sihceous" or flinty skeleton, either in 

 the form of a shell, or of detached spicules or needles ; wdiilst the 

 pseudopodia are long and thread-like, and stand out from the body 

 like rays. In this last character the Radiolaria approach very 



