144 COMPARATIVE ANATOMY 



lived and died in the waters above. At depths greater than 

 2000 fathoms, the proportion of Foraminifera in deep-sea 

 deposits becomes less and less, and at great depths the 

 deposits are largely composed of the siliceous skeletons of 

 another class of marine Rhizopoda, the Radiolaria. In the 

 greater depths the fragile calcareous shells of the Foraminifera 

 have been dissolved by the action of the carbonic acid dis- 

 solved in the sea-water ; it seems that the solvent action is 

 greater the greater the pressure. But the flinty skeletons of 

 the Radiolaria are not dissolved, and hence they take the 

 place of the Foraminifera at great depths. There are, of 

 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 Rhizopoda, 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. 



