310 



Dr. H. H. Dixon. On the Structure of 



noticeable, but to a much less degree, when coccoliths are treated with 

 a 20 per cent, solution of sodium chloride. 



On the coccosphere the coccoliths overlap each other to a consider- 

 able extent. The amount of overlap is not constant. It is usual for 

 the outer valve of one to penetrate between the outer and inner valves 

 of its neighbour, so far as to reach the collar uniting the two valves. 

 (Figs. 7, 8, 9 and 4.) In this way the coccoliths on the surface of 

 a coccosphere interlock with one another, and form a comparatively 

 rigid shell. The bevelled shape of the coccolith is evidently necessi- 

 tated by the overlapping on a curved surface. The rigidity of this 

 form of structure is best appreciated from the examination of fragments 

 of a broken coccosphere, which will preserve their curvature even 

 when they are composed of only three or four coccoliths. Such a 

 fragment is shown in fig. 2. Indeed the interlocking of the coccoliths 

 is so complete that it seems impossible to break up a [coccosphere 

 without at the same time splitting the valves of several of its coccoliths. 

 Some distortion in its shape is, however, possible without fracturing 

 the coccoliths. This yield is possibly due to the give in the outer 

 region of the valves, which appear to contain a considerable amount 

 of organic material in their composition. 



If coccospheres are dissolved in acid and subsequently or simulta- 

 neously stained, appearances are observed which seem to point to the 

 existence of an extremely fine pellicle covering over the coccoliths and 

 enclosing the whole sphere. The simplest method of demonstrating 

 this pellicle is by mixing up in glycerine jelly a stain to which a trace 

 of nitric or hydrochloric acid has been added. The coccospheres are 

 then mounted in this medium, and the solution and staining go on 

 simultaneously. Or again the material containing the coccospheres 

 may be mounted in feebly acidulated jelly, and then a drop of the 

 stain selected applied to the edge of the cover-glass. Either of these 

 methods give good results (fig. 10) if the solution is sufficiently slow. 

 The best results are obtained when the coccoliths take a fortnight or 

 more to disappear. The stains I used were acid fuchsine and methyl 

 green, fuchsine and iodine green, aniline blue, aniline green, and 

 nigrosine. 



After the solution of the coccoliths and staining it is seen (fig. 10) 

 that the coccosphere is bounded by a pellicle of extreme tenuity. In 

 the pellicle are a number of oval holes, corresponding in position and 

 number to the central depressions of the coccoliths which have dis- 

 appeared. The edges of the oval perforations are jagged, and from 

 the irregular teeth extend radial striae, which perhaps correspond to 

 the stride of the coccoliths. The striae from two adjacent perforations 

 are often continuous. The jagged teeth of the pellicle may often be 

 made out by careful focussing of the surface of the coccosphere, even 

 before it is attacked by acids and stained. Sometimes in dissolved 



