In Fusulina, and in some few other Foraminifera (cf. Fig. 310, 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 comparatively 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.
The linear dimensions of successive chambers have been {595} measured in a number of cases. Van Iterson10 has done so in various Miliolinidae, with such results as the following:
| No. of chamber | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|
| Breadth of chamber in ôç | ã | 34 | 45 | 61 | 84 | 114 | 142 | 182 | 246 | 319 |
| Breadth of chamber in ôç , calculated | ã | 34 | 45 | 60 | 79 | 105 | 140 | 187 | 243 | 319 |
Here the mean ratio of breadth of consecutive chambers may be taken as 1ôñ323 (that is to say, the eighth root of 319ã₤ãã₤34); and the calculated values, as given above, are based on this determination.
Again, Rhumbler has measured the linear dimensions of a number of rotaline forms, for instance Pulvinulina menardi (Fig. 259): in which common species he finds the mean linear ratio of consecutive chambers to be about 1ôñ187. In both cases, and especially in the latter, the ratio is not strictly constant from chamber to chamber, but is subject to a small secondary fluctuation11.