XIl] 



OF THE FORAMINIFERA 



857 



by our table on p. 791) to a constant angle of about 86°, or just 

 such a spiral as we commonly meet with in the Ammonites (cf. 

 p. 796). 



In Fusulina, and in some few other Foraminifera (cf. Fig. 424, A), 

 the spire seems to wind evenly on, with little or no external sign 

 of the successive periods of growth, or successive chambers of the 

 shell. The septa which mark off the chambers, and correspond to 

 retardations or cessations in the periodicity of growth, are still to be 

 found in sections of the shell of Fusulina, but they are somewhat 

 irregular and comjiaratively inconspicuous; the measurements we 

 have just spoken of are taken without reference to the segments or 

 chambers, but only with reference to the whorls, or in other words 

 with direct reference to the vectorial angle. 



A B 



Fig. 424. A, Cornuspira foliacea Phil.; B, Operculinu complatiata Defr. 



The linear dimensions of successive chambers have been measured 

 in a number of cases. Van Iterson* has done so in various 

 Miliolinidae, with such results as the following: 



Triloculina rotunda d'Orb. 



No. of chamber 12 3 4 5 6 



Breadth of chamber in /x — 34 45 61 



7 

 142 



8 

 182 



9 

 246 



10 

 319 



__ __ . 84 114 

 Breadth of chamber in /x, 

 calculated — 34 45 60 79 105 



* G. van Iterson, Mathem. u. mikrosk.-anat. Studien iiber Blattstellungen, nebst 

 Betrachtungen liber den Schalenbau der MilioUnen, 331 pp., Jena, 1907. 



140 187 243 319 



