Royal Microscopical Society. 71 



III. — The Silicious Dejjosit in Pinnularise. 

 By Henry J. Slack, F.G.S., Sec. E.M.S. 



(^Received during the Recess, and taken as read.') 



In making Max Schultze's artificial diatoms, all sorts of patterns 

 and gradations of size of the spherules are obtained, as the writer 

 has called attention to in a former paper. This, with other facts, 

 suggests the probability that in natural diatoms the silica may 

 always be deposited in spherules, and that what have appeared 

 plane surfaces, have the same structure as dotted ones, but on too 

 minute a scale to be discerned with the means employed. 



The uniformity of plan in the silicious deposits of diatoms is to 

 a great extent shown, and to a still further extent suggested by 

 examining one of Moller's admirable " type slides," with a good 

 immersion -|th or higher power. The gradations from large beads 

 distinctly separated, to smaller beads closely approximating, are 

 readily and instructively exhibited, so that it is easy to trace a 

 series, beginning with large forms that present no difficulty of 

 resolution, and concluding with the most delicate that tax the 

 utmost power of the optical apparatus. When beading appears 

 minute under high magnification, and each bead seems in contact 

 with its neighbour : the outline of a section made by a plane passing 

 through the bead rows perpendicular to the uppermost point of their 

 circumference, would exhibit a delicate wavy line, the depressions of 

 which would be extremely small, as they would correspond with 

 the radii of the little spheres, while the width of the curves would 

 correspond with their diameters. All that can be done under these 

 circumstances, by the best adjustments, and the most careful 

 unilateral illumination, is to exhibit minute, and often very faint, 

 alternations of light and shade, indicating rather than demonstra- 

 tively showing the character of the structm-e. When the best has 

 been done with any objective, it becomes evident that a slight 

 increase of the difficulty, from greater minuteness of the structure, 

 would render it invisible, and make the surface look plane. 



From these considerations it would be evident that if we can 

 trace the spherule structure on similar parts of a series of diatom 

 valves, and view the spherules in various gradations, fi-om com- 

 paratively large sizes to the smallest our glasses will show, we are 

 justified in supposing, if not in assuming, that similar parts of 

 diatoms in which no structure at all can be made out, really possess 

 the same structure as the preceding, though it eludes our view. 



An examination of a number of species belonging to the genus 

 Pinnularia has confirmed this view. At one time it was supposed 

 that the Pinnulariae were distinguished from allied forms by solid 

 costse replacing beaded bands. More recently this distinction has 



