jSpi.] NEW-YORK MICROSCOPICAL SOCIETY. 73 



38. The same. Outer side with torn structure, x 1750. 



29. The same. Layer immediately underneath, x 1750. 



30. The same. Showing the two layers at one view, x 1750. 



31. The same. Layer underneath. X 1750. 



32. The same. Outer layer, showing fibrils, perfect but only occasional focal images. 

 X 1750. 



33. The same. Layer underneath, x 1750. 



34. Pleurosigma sp. Single under layer of a fine species of a Pleurosigma detached 

 from the outer. At the side will be seen a part of the same valve, I believe, with the 

 both layers complete. 



35. P. angulatum. Inner side, showing curve straight from each margin and slop- 

 ing down towards the nodule and median line. This plane shows square, x 1750. 



36. The same. Same part taken at a little lower level to show hexagons and inter- 

 costals. X 1750. 



37. The same. Same side, but another valve, x 1750. 



38. The same. Outer side, showing curve straight from nodule and median line, 

 and slopes down towards the margins. N. B.— Structure on this side corresponds to No. 

 4 slide of P. formosum. X 1750. 



39. The same. Same side, showing longitudinal crack with fibril bridged across it 

 in one place, x 1750. 



40. The same. Surface abraded and two fibrils left isolated, x 1750. 



41 and 43. The same. Same spot, but at a little lower focus to show changes in 

 appearance on sound parts of valve. The changes on the surface are considerable, 

 while the fibrils themselves change but little, except to thicken. The last is the lowest 

 of the three and the appearance hexagonal. X 1750. 



43. The same. Same side, showing -where marked with a X— two fibrils joining 

 togeiher to form the regular structure, x 1750. 



44. The same. Same side— end of valve —shows isolated fibi ils in places, x 1750. 



45. The same. Same side, torn structure. X 1750. 



46. The same. Slide of P. angulatum. The specimen marked with a X is the one 

 from which the next three slides are taken. 



47. The same. Torn structure, showing free ends of fibrils. 



48. The same. Same bit taken at a little lower level to show hexagons on sound 

 parts. Note that but little change has taken place in the torn part, on accoimt of there 

 being no interference from structm-e below, x 1750. 



49. The same. Same valve, showing under layer. The part immediately under the 

 torn structure has been torn off and pushed down, leaving a considerable distance in 

 that place between the two layers, x 1'50. 



50. P. balticum. The fibrils here seem to be in long strips and thickened out at 

 regular intervals, giving the appearance, where they approach each other, of a square 

 grating. Indications also are given on the slide of the lower layer, x 1750. 



DIATOM-STRUCTURE— THE INTERPRETATION OF 

 MICROSCOPICAL IMAGES. 



BY JACOB D. COX, LL.D., F.R.M.S. 

 (^Read January 2d, 1 891.) 



Mr. Smith's paper and his photographs are a valuable and inte- 

 resting contribution to the discussion not only of diatom-struc- 

 ture, but incidentally of the interpretation and value of micro- 

 scopical images. 



