THE OCEAN WOULD. 
86 
shell. The exact maimer in which this is accomplished is doubtful > 
but the Professor thinks it probable that the soft animal has the poA*-' 1 
of diffusing its substance over the shell, and thus depositing upon 
surface additional layers of calcareous matter. 
The fossil Foraminiliera are chiefly distinguished from recent a I1( ^ 
existing species by the size of the former. While the living foriU 8 
range from one-fourth to the one-hundredth part of an inch, the tertiafljj 
strata abound in examples of Nummulites, varying from the eighth 
an inch to the size of half-a-crown. The engraving is a drawing 
from Nature, by MM. d’Archaic and Haime, of a piece of nummulfl’*' 
rock, ol Nousse, in the Landes, in which a great variety of sizes a^d 
forms are exhibited. 
The Nummulina belong to the third family, or Helicostega, ^ 
which the outer convolutions completely embrace the earlier form^ 
ones. Hence it is only by making microscopic sections, or thin sliced 
that their structure can be fully seen. When such a section is carried 
horizontally through the centre of the shell, the segments present 9 
spiral arrangement, which, like the convolutions, are remarkable 
their small size, and consequent great number. 
With respect to the distribution of the Foraminifera according ^ 
geological periods, we may briefly state that they have been found ^ 
every formation from the Silurian to the Tertiary. The species, at 
Fig- 21. Siderolites calcitrapoidcs (Lamarck). 
very simple in then forms, begin to appear in increasing numbers in 
carboniferous formations. They become more numerous, and, at ^ 
same time, more complex in their forms, in the Cretaceous period ; tb e 5 
are still more diversified and appear to have multiplied much more rapi^/ 
in the Tertiary period, where they attain the maximum of tbe jf 
numerical development. In the celebrated quarries of St. Peter, 9 
Maestrecht, the Siderolites caleitrapoides of Lamarck are found ^ 
the upper chalk (Fig. 21). In the calcareous formation of Ohaussp 
