Fig. 310. A, Cornuspira foliacea, Phil.; B, Operculina complanata, Defr.

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 Iterson[547] has done so in various Miliolinidae, with such results as the following:

No. of chamber12345678910
Breadth of chamber in µ—34456184114142182246319
Breadth of chamber in µ, calculated—34456079105140187243319

Here the mean ratio of breadth of consecutive chambers may be taken as 1·323 (that is to say, the eighth root of 319 ⁄ 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 1·187. 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 fluctuation[548].

Fig. 311. 1, 2, Miliolina pulchella, d’Orb.; 3–5, M. linnaeana, d’Orb. (After Brady.)

When the linear dimensions of successive chambers are in continued proportion, then, in order that the whole shell may constitute a logarithmic spiral, it is necessary that the several chambers should subtend equal angles of revolution at the pole. In the case of the Miliolidae this is obviously the case (Fig. [311]); for in this family the chambers lie in two rows (Biloculina), or three rows (Triloculina), or in some other small number of series: so that the angles subtended by them are large, simple fractions of the circular arc, such as 180° or 120°. In many of the nautiloid forms, such as Cyclammina (Fig. [312]), the angles