100 



THE MICROSCOPE AND ITS REVELATIONS. 



they do nearer its edge, that new lamellaB must be progressively added 

 to the surfaces of the disk, 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 tra- 

 versed by columns of non-tubular substance, which spring from the sep- 

 tal bands, and gradually increase in diameter with their approach to the 

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

 glistening tubercles. 



492. The Nummulitic Limestone of certain localities (as the South- 

 west of France, North-eastern India, etc. ) contains a vast abundance of 

 discoidal bodies termed Orbitoides (Fig. 333, c), which are so similar to 

 Kummulites as to have been taken for them, but which bear a much 



closer resemblance to Cyclo- 



F* - 34 - clypeus. These are only known 



a in the fossil state; and their 



structure can only be ascer- 

 tained by the examination of 

 sections thin enough to be 

 translucent. When one of 

 these disks (which vary in size, 

 in different species, from that 

 of a four-penny piece to that 

 of half-a-crown) is rubbed- 

 down so as to display its inter - 

 nal organization, two different 

 kinds of structure are usually 

 seen in it; one being com- 

 posed of chamberlets of very definite form, quadrangular in some species, 



Portions of the Section of Orbitoides fortisii shown 

 in Fig. 339, more highly magnified; a superficial lay- 

 er; 6, median layer. 



' 



FIG. 341. 





Vertical Sections of Orbitoides Fortisii, showing the large central chamber at a, and the median 

 layer surrounding it, covered above and below by the superficial layer. 



circular in others, arranged with a general but not constant regularity in 

 concentric circles (Figs. 339, 340, #, &); the other, less transparent, being 

 Flo 342 formed of minuter chamberlets which have 



no such constancy of form, but which might 

 almost be taken for the pieces of a dissected 

 map (a, a). In the upper and lower walls 

 of these last, minute punetuations may be 

 observed, which seem to be the orifices of 

 the connecting tubes whereby they are per- 

 forated. The relations of these two kinds 

 of structure to each other are made evident 

 by the examination of a vertical section (Fig. 



Internal Cast of portion of median 341) : which sllOWS that the portion b, FigS. 



plane of Orbitoides Fortisii, showing oo9, d40, forms the median plane, its con- 

 *a,_a',a', a f a", six cham >f cen t r j c circles of chamberlets being arrang- 

 ed round a large central chamber, as in Cy- 

 doclypeus; whilst the chamberlets of the 



portion a are irregularly superposed one upon the other, so as to form sev- 



communications; and at b 6, b' b' b 



b", portions of three annular canals. 



