306 occurrencp: of diatoms, radiolaria and infusoria, 



in the mesh-work appear to be sub-hexagonal, and the width of 

 the dark bars of the mesh-work is only a little less than the 

 diameter of the openings, viz., about 0*001 mm. 



There are from 33 to 34 meshes to the full diameter of the 

 disc. 



In the figure the frustule is magnified ( x 350 diameters). 



Diatom genus C?). — The sections of which a photograph and 

 drawing are shown on PL xviii., fig. 2, and PI. xix., fig. 1, were 

 originally mistaken hy us for a naviculoid type of diatom; we were 

 inclined at first to refer it to Nitzschia, then to Amphora. Later it 

 was noticed that in some of the sections a distinct delicate hexago- 

 nal mesh-work was visible on one or both sides. The mesh-work 

 appears to be more distinctly hexagonal than that of Coscinodiscics, 

 and the bars of the network seem to be of about the same diameter 

 as those of the latter type. The diameter of the openings in the 

 mesh-work varies from about 0*003 to 00036 mm. ; and the 

 diameter of the bars of the mesh- work is about O'OOl mm. The 

 girdle C?) view of this form shows that the hexagonal netting 

 extends completely across the ends, but it would appear that the 

 mesh-work on the girdle (1) is rather smaller than that on the 

 valve surfaces Hitherto, however, we have been unable to 

 obtain accurate measurements of their size, neither have we been 

 able to ascertain whether there is any trace of overlapping laminae 

 in what we take for the girdle zone. We believe that the sections 

 illustrated on PI xviii., fig. 2, and PL xix., fig. 1, are radial sections 

 through a biconvex form becoming slightly biconcave towards the 

 edges. The girdle (?) is a comparatively wide median zone measur- 

 ing about 001 mm. in inside diameter, and 00175 mm. in outside 

 diameter. A fact opposed to the view that the ends of the forms 

 figured represent radial sections cut through the girdle, is that in 

 many of the sections one end appears to be wider than the other. 

 In such cases, however, it is doubtful whether the specimen is 

 completely preserved so as to show the complete girdle face at 

 each end. The extreme outer diameter, measured parallel to the 

 long axis of the figure, is 0-1675 mm., and the extreme inner 

 diameter is about 0*164 mm. The diameter of this form is, 



