308 



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



seen that the oval disc, which I shall call the body of the coccolith, form- 

 ing the bottom of the central depression and carrying the single slit or 

 two D-shaped holes, is of some considerable thickness (figs. 3, 4, and 

 5). Its inner surface projects very slightly inside the inner valve, 

 and the slit or D-shaped perforations are enlarged towards its inner 

 and outer siurfaces. The whole coccolith consequently has the form 

 of a very short, thick-walled, oval tube, with its outer extremity re- 

 ciurved to form the outer valve. The inner valve is a dished collar 

 attached very close to the inner extremity of the tube. The projec- 

 tion of the tube-like portion, or body, inside the inner valve is very 

 slight, and can only be made out either by sections or by very careful 

 focussing. 



An examination of the coccoliths in plan has also revealed a few 

 points of interest. AMiere the body of the coccolith comes in contact 

 with the depressed portion of the outer valve, there are to be seen a 

 series of very minute punctations (fig. 6) corresponding, as far as could 

 be ascertained, with the radiating grooves between the striations of the 

 valve. It is possible that these pimctations represent the ends of 

 minute passages running doTvn the outside of the body and inside the 

 valves of the coccolith. Another point that may be noticed has refer- 

 ence to the material composing the transverse bar usually present, 

 dividing the slit-like canal of the body into the two D-shaped holes. 

 This bar is often ob\4ously composed of somewhat different material 

 from the rest of the coccolith, and this difference extends for some dis- 

 tance beyond the edges of the slit continuing the direction of the bar. 

 In appearance the bar and its continuation are less highly refractive 

 than the rest : and the difference of material is fiu-ther manifested by 

 the fact that it dissolves more rapidly in weak acids than the body of 

 the coccolith, while this latter goes into solution more readily than the 

 collar-shaped portion uniting the two valves. It must be borne in 

 mind, however, that this difference in the rates of solution may be in 

 part or completely due to a difference in the ease of diffusion round 

 these parts. It will be seen later that the order of solution is the 

 reverse of the order of development. The oldest parts of the coccolith 

 are the last to be dissolved. 



In a previous note Dr. Joly* and I have already noticed that there 

 is no evolution of free gas when coccoliths are attacked by acids. This 

 is probably due to the fact that the amount of gas they generate is 

 unable to overcome the cohesion of the liquid in which the reaction 

 takes place. From this consideration it would appear that the absence 

 of free gas imder the action of acids is no objection to regarding cocco- 

 liths as composed in the main of calcium carbonate. To test the 

 validity of this view I mounted some fine precipitated calcium car- 

 bonate in water, which had been boiled so that it thoroughly wetted the 

 * ' Nature,' September 16, 1897. 



