THE RHIZOPODA 137 



course, Radiolarian skeletons amongst the calcareous deposits 

 in moderate depths, but they are scarcely distinguishable 

 among the greatly preponderant mass of calcareous shells. 

 The chalk, which in some parts of England is over a 

 thousand feet in thickness, is almost entirely composed of 

 the shells of Foraminifera, and in the Barbadoes there are 

 deposits of considerable thickness formed almost entirely of 

 the flinty skeletons of Radiolaria. 



It is beyond the purpose of this book to enter into details 

 concerning all the different groups of the Protozoa, however 

 important they may be from a geological point of view. But 

 the reader should remember that animalcules whose structure, 

 except for the presence of a calcareous or flinty skeleton, is 

 scarcely more complex than that of an Amoeba, have played 

 and are still playing a very important part in the formation 

 of the earth's crust. 



