130 THE FORAMINIFERA 



80 ft, and the number of single chambers is 9 to 12. In some 

 cases the chamber which succeeds the megalosphere is consider- 

 ably larger than those which immediately follow (Fig. 56, A'). 



We thus have in this sand from the Maldives a difference in 

 size between the two forms of Heterostegina similar to that found 

 in the Nummulitic formations of the Eocene period. 1 



In the third genus of the series, represented by the species 

 Cydodypeus carpenteri, the great majority of the individuals do not 

 exceed 12 mm. in diameter, but some attain the large size of 

 64 mm. 



It is very probable, from analogy with other genera, that the 

 specimens which attain the large size are microspheric ; those of 

 smaller size are, in the specimens which I have examined, megalo- 

 spheric. 



The only specimen (Figs. 57, B, and 58) of the microspheric form 

 which I have examined is a section. 2 In it the microsphere 

 measures 29 /A, and is followed by 9 single chambers arranged 

 in a spiral. The chambers then become subdivided, as in 

 Heterostegina. After being disposed at first in a spiral, they 

 gradually extend round a larger and larger part of the circum- 

 ference of the test until they completely encircle it, and the 

 arrangement becomes annular. The twenty -fifth chamber from 

 the microsphere is, in this specimen, the first to complete the 

 circle. 



In the megalospheric form the centre is occupied by a structure 

 somewhat resembling the " primitive disc " of Orbitolites complanata 

 (p. 106). It is, however, differently constituted. The megalo- 

 sphere is very large, its average mean diameter in 9 cases being 

 245 fj., and the extremes 465 and 175 p.. It communicates by a 

 narrow neck with a large chamber which is applied to the megalo- 

 sphere for about half its circumference, and communicates in 

 turn with another large chamber. 



1 Chapman (10, p. 19) believes that he has found the dimorphic forms of Hetero- 

 stegina, and identifies them with the biconvex and the compressed varieties described 

 by Brady. He finds that the size of the full-grown megalospheric test is greater than 

 that of the microspheric, a result which he recognises as unusual. The relative sizes 

 of M aud m are said to be iu the proportion of 3:2, and I learn from Mr. Chapman, 

 by letter, that the actual diameters were 125 and 65 fj. respectively. These results 

 are so far at variance with the phenomena of dimorphism in general, and with my 

 own in this species, that it appears probable that the individuals with the smaller 

 central chamber were megalospheric specimens with rather smaller megalospheres, 

 and that Mr. Chapman did not meet witli the microspheric form. 



2 I have to thank Professor J. W. Judd for the opportunity of examining and 

 figuring this section. The specimen was obtained at Funafuti, in the Pacific, and 

 the section was prepared by Mr. Chapman, and figured by him (10, PI. III. Fig. 2) 

 on a small scale. In this paper the specimen is regarded as an unusual example of 

 the megalospheric form, but I understand, by letter, from Mr. Chapman that he is 

 now inclined to reconsider this view. Fis. 58 is prepared from a photograph of the 

 central region, on a larger scale, and Fig. 57, B, shows the arrangement of the 

 chambers more clearly. 



