64 THE DAWN OF LIFE. 



give strength, this is done by adding new portions of 

 carbonate of lime to the outside, and to these Dr. Car- 

 penter has given the appropriate name of ^^ supplemen- 

 tal skeleton ; ^' and this, when covered by new growths, 

 becomes what he has termed an " intermediate skele- 

 ton/' The supplemental skeleton is also traversed by 

 tubes, but these are often of larger size than the pores 

 of the cell-wall, and of greater length, and branched in 

 a complicated manner. (Fig. 20.) Thus there are micro- 

 scopic characters by which these curious shells can be 

 distinguished from those of other marine animals ; and 

 by applying these characters we learn that multitudes 

 of creatures of this type have existed in former periods 

 of the world's history, and that their shells, accumulated 

 in the bottom of the sea, constitute large portions of 

 many limestones. The manner in which such accumu- 

 lation takes place we learn from what is now going on 

 in the ocean, more especially from the result of the 

 recent deep-sea dredging expeditions. The Foramini- 

 fera are vastly numerous, both near the surface and at 

 the bottom of the sea, and multiply rapidly ; and as 

 successive generations die, their shells accumulate on 

 the ocean bed, or are swept by currents into banks, 

 and thus in process of time constitute thick beds of 

 white chalky material, which may eventually be hard- 

 ened into limestone. This process is now depositing a 

 great thickness of white ooze in the bottom of the 

 ocean ; and in times past it has produced such vast 

 thicknesses of calcareous matter as the chalk and 

 the nummulitic limestone of Europe and the orbitoidal 



