FORAMINIFERA. 



115 



however, extremely variable, even within the limits of a single 

 species, and it would be impossible to notice even the chief types 

 in this place. There are, however, two or three important variations 

 which may be mentioned. 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. 29, g). 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 Cristellaria 

 (fig. 23). These are the so- 

 called " nautiloid " Forami7iifera, 

 named from the resemblance of 

 the shell, in figure, to that of the 

 Pearly Nautilus. From this re- 

 semblance the nautiloid Fora- 

 minifera were originally placed in the same class as the Ammonites 

 (Cephalopoda), but their true position was shown by the examination 

 of their soft parts. In the typical nautiloid shell the convolutions 

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

 or Pidvinulina (fig. 24), the shell becomes turreted or top-shaped, 

 in consequence of the coils of the spiral passing obliquely round a 



Fig. 2 



-Cristellaria echinata, ; 

 Foraminifer. (D'Orbu 



Fig. 24.. — Pulvinulina Boueana. (D'Orbigny.) 



central axis. In other cases, as in Tinoporus, the chambers are 

 arranged in an irregular or " acervuline " manner. 



In the majority of the polythalamous Foraminifera, the successive 

 chambers of the test are so produced that the septum between any 

 two of them is formed solely by the anterior wall of the older cham- 

 ber, which thus constitutes the posterior wall of the newer one (fig. 

 25). In the highest types of the compound Foraminifer a, however, 

 each chamber is provided with its own proper wall of shell, each 

 segment, as it is produced, forming for itself a posterior wall which 

 applies itself to the anterior wall of the preceding segment ; so that 

 each septum (" septal plane ;; ) is composed of two lamellae, as seen 



