THE FORAM1NIFERA 



In the Miliolinidae, then, the tests of the megalospheric and 

 microspheric forms of a species differ in the size of the central 

 chambers, and also in some cases in the arrangement of the 

 chambers which immediately succeed it. When this difference is 

 found it depends on the fact that while the megalospheric form 

 generally grows on a uniform plan throughout, the microspheric 

 form assumes for a longer or shorter period of its growth an 

 arrangement different from that to which it subsequently con- 

 forms, and one which is, in many cases, characteristic of another 

 genus of the group. 1 



The term Initial Polymorphism was first applied by Munier- 

 Chalmas and Schlumberger to a varying condition with respect 

 to the arrangement of the initial chambers observed among 

 different individuals of the megalospheric form of the fossil 

 Idalina antiqua. It occurs also, as we shall see, in other genera. 



FIG. 27. 



The central regions of transverse sections of three examples of the megalospheric form of 

 Idalina antiqua (d'Orb.). Diameter of megalosphere in a=440, 6=400, and c=240 jx. After 

 Schlumberger. (It will be observed that the magnification of a is greater than that of 6 and c.) 



In this species the megalospheric and microspheric forms are 

 sharply contrasted in the size of their central chambers 

 (M = 180 - 440 p,, m = 12 /A), and the arrangement of the chambers 

 is, in the main, that characteristic of Biloculina. The microspheric 

 form passes through quinqueloculine and triloculine stages to a 

 biloculine condition, which, however, is converted in this genus in 

 the later stages of growth to a uniloculine state, by the lateral 

 extension of each of the chambers in turn, to embrace the whole of 

 the previously formed test. The megalospheric form begins in 

 many cases (Fig. 27, a), on the biloculine plan, to become unilocular 

 at a later stage, like the microspheric form. In some cases, 

 however, the initial chambers of this form are arranged on the 

 triloculine plan (6), and in others again on the quinqueloculine 

 (c), though the biloculine arrangement is soon assumed, in the 

 latter case with a brief intermediate triloculine phase. Moreover, 



1 I am not, however, aware of any definite form which the microspheric tests of 

 Addosina polygonia resemble. 



