594 



THE SPIRAL SHELLS 



[CH. 



86°, or just such a spiral as we commonly meet with in the 

 Ammonites* (cf. p. 539). 



In Fusuhna, and in some few other Foraminifera (cf. Fig. 

 310, a), the spire seems to wind evenly on, with httle or no 

 external sign of the successive periods of growth, or successive 

 chambers of the shell. The septa which mark oif the chambers, 

 and correspond to retardations or cessations in the periodicity of 

 growth, are still to be found in sections of the shell of FusuUna; 

 but they are somewhat irregular and comparatively inconspicuous ; 

 the measurements we have just spoken of are taken without 



A B 



Fig. 310. A, Cornusjiira foliacea, PliiL : B, Operciilinri coin phi luiia, Defr. 



reference to the segments or chambers, but only with reference 

 to the whorls, or in other words with direct reference to the 

 vectorial angle. 



The hnear dimensions of successive chambers have been 



* As von Moller is careful to explain, Naumann's formula for the "cyclo- 

 centric conchospiral " is appropriate to this and other spkal Foraminifera, since 

 we have in all these cases a central or initial chamber, approximately spherical, 

 about which the logarithmic spiral is coiled (cf. Fig. 309). In species where the 

 central chamber is especially large, Naumann's formula is all the more advan- 

 tageous. But it is plain that it is only required when we are dealing with 

 diameters, or with radii; so long as we are merely comparing the breadths of 

 successive whorls, the two formulae come to the same thing. 



