THE FORAMINIFERA 



127 



margin of the previous convolution, and have a large radial extent. 

 The aperture is crescentic and undivided, but is supplemented 

 by pores distributed along the 

 septum. 



The canal system is highly 

 developed, a plexus of canals, 

 in connection with the meri- 

 dional vessels, running in a 

 keel -like thickening at the 

 outer rim of the test. 



A sample of sand obtained 

 by Mr. J. Stanley Gardiner 

 from Suvadiva in the Maldive 

 Archipelago consists for the 

 most part of the tests of Oper- 

 culina complanata and Hetero- 

 stegina depressa, d'Orb. Of the 

 former I have found two speci- 

 mens of the microspheric form, 

 in both of which m = 27 ^ and 

 the second chamber is very 

 minute (Fig. 55, B). 



In five specimens of the meg- 

 alospheric form the values of 

 M vary from 45 to 63 /* and 

 have an average of 54 p. 



I have not observed that 

 the tests of the two forms of 

 this species are distinguished 

 by a difference in size. 



In Nummulites the chambers are of very small extent in a 

 radial direction (Fig. 6), so that each convolution adds little to the 

 diameter of the test, and the outline of the latter is nearly circular. 

 The number of the convolutions is however very large. The 

 alar prolongations, on the other hand, are highly developed, and 

 extend nearly to the centre. As the chambers of each successive 

 convolution are thus produced, the result is that the tests are 

 strongly biconvex, the spiral axis measuring from one -third to 

 three-quarters of the diameter. The alar prolongations of the 

 chambers may be directed straight towards the spiral axis (radiate 

 type) or take a sinuous course (sinuate type), or they may be 

 replaced by a number of separate chamberlets forming when 

 exposed in worn specimens a network over the surface (reticu- 

 late type). The aperture and canal system resemble those of 

 Operculina.' 



The microspheric form attains, as we have seen, a much 



Fio. 55. 



Operculina complanata, Def. in, a complete 

 test, x 8 ; A, central part of section of megalo- 

 spheric form ; B, of the microspheric form. (A 

 and B, x 50.) 



