NUMMULITES 



835 



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 

 commencement, its chamberlets being arranged in rings around a 

 central chamber. This remarkable genus, at present only known in 

 the recent condition by specimens dredged at considerable depths 

 from the coast of Borneo and at bne or two points in the Western 

 Pacific, is perhaps the largest of existing Foraminifera, some speci- 

 mens of its discs in the British Museum having a diameter of two 

 and a quarter inches. Notwithstanding the difference of its plan 

 of growth, it so precisely accords with 

 the Nummuline type in every cha- 

 racter which essentially distinguishes 

 the genus that there cannot be a 

 doubt of the intimacy of their rela- 

 tionship. It will be seen from the 

 examination of that portion of the 

 figure which shows Cycloclypeus in 

 vertical section that the solid layers 

 of shell by which the chambered por- 

 tion is inclosed are so much thicker, 

 and consist of so many more lamellae 

 in the central portion of the disc 

 than they do nearer its edge, that 

 new lamellae must be progressively 

 added to the surfaces of the disc 

 concurrently with the addition of new 

 rings of chamberlets to its margin. 

 These lamellae, however, are closely 

 applied one to the other without any 

 intervening spaces ; and they are all FIG. 636. Section of Orlitoldcs 

 traversed by columns of non-tubular Fortisii, parallel to ^ the surface, 

 substance, which spring from the 

 septal bands, and gradually increase layer.' 

 in diameter with their approach to 

 the surface, from which they project in the central portion of the 

 disc as glistening tubercles. 1 



The Nummulitic limestone of certain localities (as the south-west 

 of France, Southern Germany, North-Eastern India, &c.) contains a 

 vast abundance of discoidal bodies termed Orbitoides (fig. 630, B), 

 which are so similar to Nummulites as to have been taken for them, 

 but which bear a mueh closer resemblance to Cycloclypeus. These 

 are only known in the fossil state ; and their structure can only be 

 ascertained by the examination of sections thin enough to be trans- 

 lucent. When one of these discs (which vary in size, in different 

 species, from that of a fourpenny-piece to that of half a crown or 

 even larger) is rubbed down so as to display its internal organisation. 



1 Dr. L. Rhumbler's ' Entwurf eines natiirlichen Systems der Thalamophoren ' 

 (Nachr. Ges. Gottingen. 1891, p. 51) is chiefly based on palaeontological considerations. 



3 H 2 



