114 



THE FORAMINIFERA 



In the microspheric form (Fig. 43, B and b) m= 15-18 /^, and is 

 followed by some five or six chambers arranged in a spiral before 

 the alternating arrangement is assumed. One specimen contains 

 at least 13 nuclei. 



In the megalospheric form (Fig. 43, A), the average mean dia- 

 meter of the megalosphere is about 60 /* (the limits of those 

 measured were 44 and 72 /z). The initial spiral is here somewhat 

 shorter, consisting of four chambers. A single large nucleus is 

 seen in these specimens some distance along the alternating set of 

 chambers. 



The spiral arrangement of the early chambers is much more 

 conspicuous in Spiropleda annectens, P. and J. (Fig. 44, A, B, and b). 



Fio. 44. 



A, B, and b, Spiroplecta annectens, P. and J ; A, the megalospheric, B, b, the microspheric 

 form. C, Verneuilina pygmaea, Egger. D, Bigerterina robusta, Brady. E, Clavulina angularis, 

 d'Orb. A and b, x 70, original ; B-E, from Brady (3). 



What appears to be the megalospheric form of this species has 

 long been known as a Cretaceous fossil. The species occurs at the 

 present day round the coasts of Australia, and has been recognised 

 in sand from the Malay Archipelago by Mr. F. W. Millett (24, 

 Part VII., 1900), to whom I am indebted for calling my attention 

 to the evidence of dimorphism in this species, and for the oppor- 

 tunity of examining the specimens from which the following details 

 are given. 



Among six specimens of the megalospheric form (Fig. 44, A), the 

 average value of M = 60 yu, (the limits of variation are 53 and 71 /*), 

 and one nearly complete spiral whorl of chambers intervenes before 

 the straight and biserial part of the test begins. 



The microspheric form attains a larger size. Among 15 speci- 

 mens, the average value of m= 17 p (the limits being 11 and 20 //,), 



