THE FORAMINIFERA 99 



In flabelliformis the value of M is less, though the average in 

 my specimens is not below 64 p. 



In the microspheric form the average value of m in six cases is 

 18 /*, the highest being 21 p, and the lowest 16 p. 



In the megalospheric form the megalosphere is followed by a 

 spiral passage reaching round |-f of its circumference, and both 

 megalosphere and spiral 

 passage frequently ex- 



hibit the perforated con- ,.-..... 



dition found in Peneroplis ...-??' Vo~s ' ^**h"ti__ 



(Figs. 31, A, and 32). ,,< o ^ 



The first of the spiral , : \; 



series of chambers is S^HB '.vja-'P^' 



usually simple, but it L-^^' 



frequently opens into its 

 successor by two aper- 

 tures, and in the second Flo 3 ., 



the transverse ribs gener- OrUculina adunca, central part of a section of a 



ilhr maVo tViair armo-ar megalospheric test passing transverse to the median 

 appear- p , a * e jr meg alosphere ; sp.c, spiral passage. 



ance, which, becoming 



more marked in the following chambers, subdivide them into 

 chamberlets. The chambers also communicate with one another 

 by an increasing number of apertures, arranged in several rows 

 along their peripheral walls. 



In the microspheric form of Orbiculina, as in that of Peneroplis, 

 the microsphere opens direct into the first of the spiral series of 

 chambers, and in this form there are generally some twenty simple 

 chambers communicating by a single aperture before the sub- 

 division into chamberlets begins (Fig. 31, B and b). 



Such are the characters of well-grown specimens of Orbiculina; 

 but on examining the construction of the tests of small specimens, 

 as displayed in section, a mode of variation of a different kind 

 becomes apparent, one which illustrates the phenomenon of Initial 

 Polymorphism described by Munier-Chalmas and Schlumberger 

 in Idaliiia. In the sample of sand which furnished the varieties 

 above described were numbers of small megalospheric specimens 

 resembling the young of the typical forms (c and / in Fig. 30), 

 but beginning in a megalosphere of small size. In the specimen 

 represented in Fig. 31, A', the central part of one of these is seen 

 in section. Here the megalosphere measures only 34 p. in diameter. 

 Associated with the small size of the megalosphere of these forms 

 is a long series of single chambers before the subdivision into 

 chamberlets begins. In both characters they thus vary in the 

 direction of the microspheric form, though always distinguishable 

 from it by the presence of the spiral passage. In Orbiculina then, 

 as in Idaliiia, the construction of the early part of the test is 



