CHALK MAKERS, OR FORAMINIFERA. 4 



elusion that the greater thickness of the earlier and 

 older growths of the shell is owing to the circum- 

 stance that, on the addition of each new segment,- the 

 cell-wall by which it is enclosed has not terminated 

 at the boundary of the individual segment itself, but 

 has been prolonged over a considerable number, if 

 not the whole, of the segments belonging to the 

 outermost convolution of the animal. But before the 

 newly-formed segment enclosed itself within the 

 limits of its own calcareous shell, it must have had 

 the power of spreading itself out in the form of a thin 

 gelatinous layer, investing the whole of the pre- 

 existing organism." l That is to say, whenever the 

 animal has so increased in size, in the last-formed 

 chamber, that a new chamber must be constructed, 

 the sarcode diffuses itself over the whole external 

 surface of the shell, and deposits a new layer of 

 calcareous matter over all the shell, then retreats 

 within the last segment again, and completes a new 

 segment, which becomes the last, and is the thinnest, 

 because it has had no external stratum deposited 

 upon it. Moreover, it is known that, at certain times, 

 the sarcode does envelope the entire shell, and this 

 explains wherefore it is done. 



This hypothesis explains, and accounts for, the 



1 "Transactions of the Microscopical Society of London," 

 vol. iii. (1850) p. 105. 



