FORAMINIFERA. 93 



The sixth family differ entirely in appearance and structure from the 

 other Foraminifera. They are more opaque than the other orders, 

 having a resemhlance to white porcelain, which presents a rich amber- 

 brown hue when viewed by transmitted light. They are more or less 

 oblong, each new segment being nearly equal to the entire length of 

 the shell, so that the terminal orifice presents itself alternately at its 

 opposite extremities, sometimes in one uniform plane, as in Spirolocu- 

 lina (Fig. 20), and Faujasina (Fig. 21). At other times each new seg- 

 ment, instead of being exactly opposite each other, is a little on one 

 side. 



Professor Williamson has shown that the 

 shell enclosing each new segment is at first 

 very thin; but as additional calcareous 

 chambers are formed, each addition not only 

 encases the new gemmation of the soft 

 animal, but extends over all the exterior of 

 the previously formed shell. The exact 

 manner in which this is accomplished is 

 doubtful ; but the Professor thinks it prob- 

 able that the soft animal has the power of Fi s 21 - 

 diffusing its substance over the shell, and thus depositing upon its 

 surface additional layers of calcareous matter. 



The fossil Foraminifera are chiefly distinguished from recent and 

 existing species by the size of the former. While the living forms 

 range from one-fourth to the one-hundredth part of an inch, the tertiary 

 strata abound in examples of Nummulites varying from the eighth of 

 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 nummulitic 

 rock, of Nousse, in the Landes, in which a great variety of sizes and 

 forms are exhibited. 



The Nummulina belong to the third family, or Helicostega, in 

 which the outer convolutions completely embrace the earlier-formed 

 ones. Hence it is only by making microscopic sections, or thin slices, 

 that their structure can be fully seen. When such a section is carried 

 horizontally through the centre of the shell, the segments present a 

 spiral arrangement, which, like the convolutions, are remarkable for 

 their small size, and consequent great number. 



With respect to the distribution of the Foraminifera according to 



