638 



MICROSCOPIC GEOLOGY. 



still Foraminiferous, are formed upon a plan of structure altoge- 

 ther different. On account of the minuteness of their parts, and 

 the completeness of their fossilization, their structure can only 

 be elucidated by sections thin enough to be examined by the 

 microscope with transmitted light ; and it is consequently to the 

 assistance afforded by this instrument, that we are indebted for 

 our knowledge of the curious type of organization which it pre- 

 sents. "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 internal organization, two 

 different kinds of sti'ucture are usually seen in it; one being 

 composed of chambers of very definite form, quadrangular in 

 some species, circular in others, arranged with a general but not 

 constant regularity in concentric circles (Figs. 340, 341, 1, 1) ; the 

 other, less transparent, being formed of minuter cells which have 

 no such constancy of form, but which might almost be taken for 

 the pieces of a dissected map (Figs. 340, 341, a, a). In the upper 



Fig. 341. 



Ponions of the same section, more highly magnified : — a, superficial layer ; 6, median layer. 



and lower walls of these last, minute punctations may be ob- 

 served, which seem to be the orifices of connecting tubes whereby 

 they are perforated. The relations of these two kinds of struc- 

 ture to each other, are made evident by the examination of a 

 vertical section (Fig. 842) ; which shows that the portion a, Figs. 



FlO. 342. 



Vertical Section of Orbitoides Prattii, showing the large central cell at a, and the median layer 

 surroujidiiig it, covered above and below by the stiperficial layers. 



340, 341, forms the central plane, its concentric circles of cells 

 being arranged round a large central cell a, as in Orbitolite (Fig. 

 206) ; whilst the cells of the portion b are irregularly superposed 

 one upon the other, so as to form several layers, which are most 

 numerous towards the centre of the disk, and thin away gradu- 

 ally towards its margin. By the perforations in these layers, the 

 pseudopodia proceeding from the central plane of chambers may 

 have found their way direct to the surface, or at any rate would 

 have been brought into connection with the segments lying 

 nearer to it. No organisms precisely resembling the Orbitoides, 



