72 JOURNAL OF THE [April, 



LIST OF LANTERN SLIDES REFERRED TO IN THE PAPER "ON THE STRUC- 

 TURE OF THE PLEUROSIGMA VALVE." 



BY T. P. SMITH, F.R.M.S. 



1. P. formosum. Inner side of valve seeming to consist of a square grating, in 

 •which, when perfect, a focal image is formed in the alternate squares. X 1750. 



2. The same. Same side, but another valve -one of the sides, from the margin 

 inward, shows appearance when perfect. X 1750. 



». The same. Layer immediately under No. 4 on the outer side of valve, and seems 

 to consist of a robust diagonal grating. X 1750. 



4. The same. Layer immediately over No. 3, above the median line. The under 

 structure has sunk down, leaving the long triangular bit adhering to the cover, x 1750. 



5. The same. Layer on the outer side of the valve, sound, x 1750. 



6. The same. The diagonal grating immediately under, sound. X 1750. 



7. The same. Same layer as No. 5. but images showing in patches. 



8 The same. Grating immediately under showing cause of patchiness. The holes 

 in under gratmg are either some of them stopped up, or all the parts of the grating are 

 not on the same level. 



9. The same. Ultimate structure of layer No. 4, consisting of short bars of silex 

 arranged as an irregular fibril. X 1750. 



10. The same. Ultimate structure arranged on the valve, but stripped off in places. 

 The diagonal lines represent the No. 3 grating underneath, out of focus. Note how the 

 bright focal spots are caught between the fibrils. X 1750. 



11. The same. Same structure arranged to form squares around the nodule. It 

 will be seen that in some places the appearance is that of a square grating with well- 

 defined cross-bars, but when the fibrils diverge from each other the cross-bars are lost. 

 I am inclined to think the cross-bars an optical illusion. X 17 iO. 



12. The same. Inner side of valve, same as No. 1, magnified 875 times only, that 

 the detail may appear of the same size as the smaller species, such as P. angulntum. 

 This valve is perfect and does not show the grating except at one edge. On focussing 

 inward all the white interspaces form into green "beads." X 875. 



13. The same. Outer layer, same as No. 4, perfect. Median line appears at the 

 edge, but on focussing down that part of the valve is seen shelving down, showing the 

 outer side of the valve to be V-shaped in section. X 875. 



14. The same. Torn structure from same side, some of which has floated off from 

 the valve altogether. Note the diagonal lines underneath. X 875. 



15. P. angulatum (?)• Same side, but another valve. Note fibril where marked 

 with a cross, to compare with same structures on finer specimens. X 875. 



16. P. decorum. Outer side. Underneath is the same diagonal grating, which I 

 have not figured. X 1750. 



17. Pleurosigma sp. Small species shown where marked with a X. This is the 

 species from which the next nine slides are taken. 



18. The same. Under layer, with a hole clean through in one part. X 1750. 



19. The same. Same valve, outer layer, with fibrils projecting over part of hole. 

 X 1750. 



30. The same. With torn fibrils, outer layer. X 1750. 



21. The same. Under layer of same valve, showing long strip and hole torn out, 

 which the fibrils shown on No. 20 have bridged over. X 1750. 



22. The same. With bit chipped off the edge and fibrils lying loose. X 1750. 

 28. The same. With fibrils imravelling. x 1750. 



24. The same. Another part of same valve, x 1750. 



25. The same. Surface abraded, and showing fibrils on parts of valve. X 17.50. 



26. The same. Inner side of valve, corresponding to No. 1 layer of P. formosum. 

 X 1750. 



27. Pleurosigma sp Specimen marked with a X, and the same species from which 

 the next six slides are taken. 



