514 FORAMINIFERA I HETEROSTEGINA ; CYCLOCLYPEUS. 



Fig. 261. 



ropUs would be converted by the like subdivision into an Orbiculina 

 (§ 374). Moreover, we see in Heterostegina, as in Orbiculina, a 

 great tendency to the opening-out of the Spire with the advance of 



age ; so that the 

 apertural margin 

 extends round a 

 large part of the 

 Shell, which thus 

 tends to become 

 discoidal. And it is 

 not a little curious 

 that we have in 

 this series another 

 form, Cycloclypeus, 

 which bears exactly 

 the same relation 

 to Heterostegina, 

 that Orbitolites does 

 to Orbiculina ; in 

 being constructed 

 upon the cyclical 

 plan from the com- 

 mencement, its 

 chamberlets being 

 arranged in rings 

 around a central 

 chamber (Plate xvi., fig. 1). This remarkable Genus, at pre- 

 sent only known by specimens dredged up from considerable 

 depths off the coast of Borneo, is the largest of existing Forami- 

 nifera; some specimens of its Disks in the British Museum 

 having a diameter of 1\ inches. Notwithstanding the dif- 

 ference of its plan of growth, it so precisely accords with the 

 Nuuimuline type in every character which essentially distinguishes 

 that Genus, that there cannot be a doubt of the intimacy of their 

 relationship. It will be seen from the examination of that por- 

 tion of the figure which shows Cycloclypeus in Vertical Section, 

 that the solid layers of Shell by which the chambered portion is 

 enclosed are so much thicker, and consist of so many more Lamellae, 

 in the central portion of the disk, than they do nearer its edge, 

 that new lamellae must be progressively added to the surfaces of 

 the disk, concurrently with the addition of new rings of Chamber- 

 lets to its margin. These Lamellae, however, are closely applied one 

 to the other, without any intervening spaces ; and they are all 

 traversed by Columns of non-tubular substance, which spring from 

 the septal bands, and gradually inci'ease in diameter with their 

 approach to the surface, from which they project in the central 

 portion of the disk as glistening tubercles. 



395. The Nummulitic Limestone of certain localities (as the 



Heterostegina. 



