62 



PROTOZOA. 



by the shell depending wholly upon this. The forms, however, 

 assumed by the shells of Foraminifera are extremely variable, 

 even within the limits of a single species, and it would be im- 

 possible to notice even the chief types in this place. There 

 are, however, two or three important variations which may 

 be noticed. If the buds are thrown out from the primitive 

 spherule in a linear series so as to form a shell composed of 

 numerous chambers arranged in a straight line, we get such 

 a type as Nodosaria (fig. 6, e). When the new chambers are 

 added in a spiral direction, each being a little larger than the 

 one which preceded it, and the coils of the spiral lying in the 

 same plane, we get such a form as Discorbina (fig. 6, c), or 

 Robulina (fig. 8). These are the so-called " nautiloid " Fora- 



minifera, from the resem- 

 blance of the shell in figure 

 to that of the Pearly Nauti- 

 lus. From this resemblance 

 the nautiloid Foraminifera 

 were originally placed in the 

 same class as the Ammonites 

 ( Cephalopoda), but their true 

 position was shown by the 

 examination of their soft 

 P*s. In the typical nauti- 

 loid shell the convolutions 

 of the spiral all lie in one plane ; but in other cases, as in 

 Rotalia (fig. 9) the shell becomes turreted or top-shaped, in 

 consequence of the coils of the spiral passing obliquely round 

 a central axis. In other forms, such as Nummulites (fig. 13), 



Fig ' 8 -- 



Fig. 9. Rotalia Boueana. D'Orbigny. 



Orlitolites, and Orbitoides, the shell, though consisting essen- 

 tially of a succession of chambers arranged in a spiral series, is 

 of a much more complex nature. Lastly, in addition to these 

 symmetrical forms, there are others, such as Globigerina (fig. 6, 

 /), in which the arrangement of the segments is very irregular. 

 Remains of Foraminifera have been found in all the great 



