68 



THE AMERICAN MONTHLY 



[April, 



transparent film on a black back- 

 ground), but the hexagonal outlines 

 where the hexagonal walls were bro- 

 ken away, and the central circular 

 areolae were still to be seen with care- 

 ful looking, I then turned to the 

 thicker part of the shell, and here 

 came an unlooked-for surprise. I 

 immediately saw that there were two 

 classes of appearances to be exam- 

 ined. 1st. In small patches over the 

 surface from which the upper lamina 

 had been removed the hexagonal 

 walls stood up here and there like 

 islands. These walls were evidently 

 thickened and incrusted with a white 

 substance apparently more porous 

 than the silex, and this incrustation 

 took the form of nodules at the angles 

 of the hexagons, whilst it partly filled 

 the hexagonal cell at the bottom, giv- 

 ing it a hemispherical or cup-like 

 form. 2d. Beyond the general line 

 marking the fracture and removal of 

 the upper lamina, and where it was 

 still in place, the surface was smooth 

 and in all respects of the same ap- 

 pearance as the lower surface seen on 

 the first specimen. This I repeated 

 and re-examined till I felt sure of my 

 observations, and that there was no 

 illusion about it. Three classes of 

 appearances stood there as opaque 

 objects, too clear for question: ist, 

 the black, lower lamina with faint 

 hexagonal and circular markings ; 2d, 

 the island-like portions of the hexag- 

 onal cells without the upper film, 

 and incrusted with the white sub- 

 stance ; and 3d, the upper lamina 

 surface, smooth and gray, with its 

 darker hexagonal tracing and circles 

 within. 



' But it occurred to me to add another 

 test. Whilst the surface was still illu- 

 minated by the vertical illuminator I 

 threw a beam of light through the 

 achromatic condenser from the mirror 

 below, and now had what seemed 

 demonstrative evidence, making as- 

 surance doubly sure. The lower film 

 was plainly seen, very thin, with shal- 

 low circular areolation, the hexago- 

 nal lines being almost invisible ; the 



patches of cell-structure stood out viv- 

 idly, less changed than the rest ; but 

 the imbroken part of the structure with 

 both lamina; in place, made transpa- 

 rent by the strong, transmitted, bluish 

 light (the condenser had a blue mod- 

 erator) , showed the internal structure 

 exactly as in the island patches, whilst 

 the fainter red beam of light from the 

 vertical illuminator still marked the 

 gleam of the upper surface by reflec- 

 tion, and the whole structure stood 

 revealed. By turning on and oft' the 

 transmitted light from the mirror the 

 surface view or the internal structure 

 could be seen in turn, and the fasci- 

 nating experiment was repeated again 

 and again.' 



We have not, however, exhausted 

 the evidence which the transparent 

 valves can give us. MM. Prinz and 

 Van Ermengem say that the centre of 

 the ' eye-spot,' viewed by transmitted 

 light, never shows any film. It is true 

 that along a bi'oken margin of a sepa- 

 rated inner lamina of Coscinodiscus 

 the eye-spot is usually found empty ; 

 but this is not always so, and in the 

 unbroken portions of such a plate 

 proper attention to the correction of 

 the objective will enable us to detect 

 it in the robust shells found in the 

 Nottingham earth. If it were not so 

 it would not be strange, for the color 

 test proves that the film is of wonder- 

 ful thinness ; and that it should be 

 broken out by any force sufficient to 

 sepai-ate the laminae of the valve or 

 break it across would be natural 

 enough. If a sash made up of small 

 panes of glass were broken in two we 

 shouldly hardly expect to find many 

 whole panes sticking out of the rem- 

 nants of the wooden frame. The evi- 

 dence got from the examination of the 

 shells as opaque objects, with both low 

 powers and the vertical illuminator, 

 would hardly be weakened by the ab- 

 sence of the central part of the ' eye- 

 spots.' 



In the Nottingham slides all the 

 parts of the gigantic discs are in- 

 creased in size and thickness, and 

 upon examining the interior plate we 



