THE FOR A MINI PER A 



microspheric form (Fig. 24, b) the chambers succeeding the micro- 

 sphere are arranged on the quinqueloculine plan, and this arrange- 

 ment is maintained during the addition of a smaller or larger 

 number of chambers, according to the species. At a certain stage 

 the chambers become wider, and conform to the triloculine plan. 

 Finally, the biloculine arrangement is assumed and maintained 

 during the remainder of growth. 



In like manner the megalospheric forms of Triloculina are built 



Fm. 24. 



Biloculina ilepressa, d'Orb. Transverse sections, a, of the megalospheric form, x 50. 6, 

 of the microspheric form, x 90. (Two external chambers have been omitted in 6.) (After 

 Schlumberger, 55.) 



I am indebted to the Cambridge Philosophical Society for permission to use the block from 

 which these figures are prepared. 



up on the triloculine plan throughout, while the microspheric forms 

 begin life on the quinqueloculine plan, though they conform early 

 to the triloculine (Fig. 25). 



In some species it appears that a difference in the arrangement of the 

 chambers is maintained throughout the growth of the test. Thus in 

 Biloculina lucernula, Schwager, the microspheric form, commencing on the 

 quinqueloculine plan, becomes triloculine, but appears never, according to 

 Schlumberger, to attain to the biloculine arrangement, which the megalo- 

 spheric form follows throughout. The two forms are shown to belong to 

 the same species by similarities in the shape of the chambers, and also 



