N. E. BROWN ON THE STRUCTURE OF DIATOMS. 337 



y^th oil-immersion of N.A. 1*3, on account of its greater 

 magnification, as it is really a xjth, not a true y^th. 



Description of Plate 23. 



Fig. 1. Part of one of the outer films of the outer grating of 

 Pleurosigma balticum, x 3,000. The central part from a camera- 

 lucida drawing, the remainder added to scale from various parts 

 of the films, to show the manner in which the bars project and 

 are held in place by the pore-perforated membrane of silex. 

 Realgar mount, central light, no stop. 



Fig. 2. Part of a curved bar from a partly dissolved specimen 

 of Pleurosigma balticum, which presents both dorsal and edge 

 views, drawn as seen, to a scale of about 9,000 diameters. 

 Realgar mount, central light, no stop. 



Fig. 3. Part of the grating of Swrirella gemma, x 3,000. 

 Realgar mount, central light, no stop. 



Fig. 4. Four meshes of the same enlarged to the scale of 9,000 

 diameters. 



Fig. 5. Fragment of the film overlaying the outer grating of 

 Pleurosigma angulatum, X 3,000. Realgar mount, Leitz dark- 

 ground illuminator, without a funnel-stop at the back of the 

 objective. 



Fig. 6. Diagrammatic enlargement of the bars of the film 

 over the outer grating of P. angulatum, to show the manner in 

 which they appear to overlie one another, drawn to a scale 

 of 6,000 diameters. No pores could be seen when this appear- 

 ance is visible. 



Fig. 7. Outer and inner grating of P. angulatum under the 

 film of diamond-shaped meshes, x 3,000. Realgar mount. 



Fig. 8. Two meshes of the same enlarged to 9,000 diameters. 



Fig. 9. Outer grating of P. angulatum, seen at the focus 

 immediately preceding the hexagonal appearance of fig. 7, 

 x 3,000. Realgar mount. 



Fig. 10. Fragment of the grating of Xavicula serians, x 3,000. 

 Picric-piperine mount ; upper part showing the coarse inner 

 grating, as seen with central light and a central stop in the con- 

 denser, green screen ; lower part showing the outer grating 

 superposed upon the coarser grating, as seen illuminated by a 

 Leitz dark-ground illuminator^ 



