9 8 



THE FORAMINIFERA 



chambers, are shown by section to be microspheric, though in their 

 younger stages the microspheric forms are not readily distinguished 

 by external characters. In a set of 38 examples of this variety, 

 of such a size that the microspheric forms were not externally 

 differentiated, 34 were found, by section, to be megalospheric and 

 4 microspheric, a proportion of 1 to 9'5. The diameter attained by 

 the megalospheric form of this variety is, in my examples, 4 mm. 



S/t./L 



_ _ 



Fio. 31. 



Central regions of sections of Orbiculina adiinca in the median plane. A, large, A', small 

 xample of the megalospheric form. B, microspheric form, x 93. b, centre of B, x 250. sp.p, 

 spiral passage. 



I have failed to find a single microspheric form of the variety 

 compressa ; 100 examples, examined in section, belong to the 

 megalospheric form. Among 24 examples of the variety flabelli- 

 formis, however, one appears to be microspheric. 



The diameters of the megalospheres in well-grown specimens 

 of the varieties adu-nca and compressa are as follows : 



Var. adunca . 

 ,, compressa 



No. of Specimens 

 examined. 



28 

 32 



Highest 

 Value of AT. 



146 M 

 155 /x 



Lowest 

 Value of AT. 



81 M 

 70 M 



Average. 



117 fi 

 109 /* 



