XIl] 



OF THE FORAMINIFERA 



865 



Firstly we have the typically spiral shells, which occur in great 

 variety, and which (for our present purpose) we need hardly describe 

 further. We may merely notice how in certain cases, for instance 

 Globigerina, the individual chambers are little removed from spheres; 

 in other words, the area of contact 

 between the adjacent chambers is 

 small. In such forms as Cyclamynwa 

 and Pulvinulina, on the other hand , 

 each chamber is greatly overlapped 

 by its successor, and the spherical 

 form of each is lost in a marked 

 asymmetry. Furthermore, in Glo- 

 higerina and some others we have 

 a tendency to the development of 

 a gauche spiral in space, as in so 

 many of our univalve molluscan 

 shells. The mathematical problem 

 of how a shell should grow, under 

 the assumptions which we have 

 made, would probably find its most 

 general statement in such a case as 

 that of Globigerina, where the whole 

 organism lives and grows freely 

 poised in a medium whose density 

 is little different from its ow^n. 



The majority of spiral forms, 

 on the other hand, are plane or 

 discoid spirals, and we may take it 

 that in these cases some force has 

 exercised a controlling influence, so 

 as to keep all the chambers in a 

 plane. This is especially the case in 

 forms like Rotalia or Biscorhiiui 

 (Fig. 430), where the organism lives 

 attached to a rock or a frond of sea- weed ; for here (just as in the case 

 of the coiled tubes which little worms such as Serpula and Spirorbis 

 make, under similar conditions) the spiral disc is itself asymmetrical 

 its whorls being markedly flattened on their attached surfaces. 



m:Wt^- 



"Fig. 430. Discorbina bertheloti d'Orb. 



