128 THE FORAMIN1FERA 



greater size than the megalospheric (Fig. 5). I am not aware of 

 any record of the actual size of the microsphere. 



In the megalospheric forms M is very large, attaining in some 

 cases 1 mm. 



While Operculina is closely connected on the one hand with 

 Nummulites, it forms, on the other, the simplest term of a series 

 Operculina, Heterostegina, Cycloclypeus, which presents among 

 the Nummulitidea a remarkably complete parallel to the Peneroplis, 

 Orbiculina, Orbitolites series in the Miliolidea. 



In Heterostegina (Fig. 56) the arrangement of the chambers is 

 spiral, though they become somewhat more embracing as age 

 advances. The chambers which are first formed are simple, as 

 in Operculina, but they soon become subdivided by partitions which 

 are disposed perpendicularly to the plane of the spiral, and, on the 

 whole, transverse to the long diameter of the chambers. The 

 chambers are thus subdivided into more or less quadrangular 

 chamberlets. As in Operculina, the chambers of the inner convolu- 

 tions are produced into alar prolongations (which are, however, 

 not subdivided into chamberlets), while the later chambers are 

 simply applied to the edge of the preceding convolution. The 

 arrangement thus presents considerable resemblance to that of 

 Orbiculina, but in addition to the presence of the canal system, 

 perforate walls, and double septa, there is also a marked difference 

 in the mode of communication of the chamberlets, for here the 

 adjacent chamberlets of a chamber do not communicate directly 

 with one another, but each communicates as a rule with two 

 chamberlets of the preceding and with two of the succeeding 

 chambers. (These communications are not displayed in the 

 sections figured, but may be readily seen in the protoplasmic 

 casts of decalcified specimens.) 



The canal system is well developed and resembles that of 

 Operculina, a marginal plexus being present here also. 



In the sample of sand from the Maldive Islands, above men- 

 tioned, the great majority of the specimens of Heterostegina range 

 from a small size up to about 4 mm. in their larger diameter (Fig. 

 56, A), but a few far exceed the rest, attaining a diameter of 10 

 or more mm. (Fig. 56, B). I am unable to recognise any differ- 

 ence in the external appearance of the two forms, beyond that 

 in size, and the peculiar shape of the large specimens caused 

 by the greater width of the terminal convolution. On making 

 sections of the tests it is found that the large specimens are 

 microspheric and the smaller ones megalospheric. 



In two specimens of the former m = 27 p in both, and a spiral 

 of some 36 simple chambers succeeds before the septa appear, 

 dividing the chambers into chamberlets (Fig. 56, B'). 



In the megalospheric form M varies in four cases from 70 to 



