THE DEPOSITS OF THE SEA-BOTTOM. 



85 



ring, inside of this a more luminous one, and inmost a very strongly luminous line formed by the 

 rays of the central ring, which are all thrown towards the middle; that we get a line and not a point 

 is the consequence of the elliptic form of the central ring; it is likely to be this line that is found in 

 many figures as a central grain, but perhaps also other forms of Coccoliths may be found really pro- 

 vided with such a central grain. If the tube is lowered farther down (to/) the two outer circles will 

 almost unite into one, while the inner line will form a more indefinite, circular spot in the middle; 

 still farther down (at g) this spot has disappeared, and only one, indistinct circle is seen. 



These pictures of rays are seen most distinctly, when the Coccolith is placed in water; in 

 Canada balsam they are developed in the same way, but to a far slighter degree. It is of no con- 

 sequence which side of the Coccolith is turned up; but if, on the other hand, it was placed in a fluid 

 with larger refraction than calcite, the whole figure would be inverted. In consequence of the 

 above stated rule with regard to the refraction we may judge from this system of shades that the 

 central ring is much more strongly refracting than the inner and outer part of the Coccolith; if the 

 whole body is composed of calcite the difference in refraction must arise from the thickness of the 

 place in question. It is not to be seen whether the central sphere or the marginal plate is the more 

 refracting; but we may come to the conclusion that no more strongly refracting central grain is found 

 in the middle, as such a one would produce a white shade in the middle of the field of view, at line 

 a. If an outer ring were found, it must form a shade in the same field of view outside the outer 

 ring; but there is no trace of such a shade. By this examination we consequently get the result that 

 the Coccoliths, or at least those examined by me, consist only of the above mentioned three parts : 

 the central sphere, the central ring, and the marginal plate. 



The Coccospheres, as far as discernible, are globular bodies of on average diameter of 20 /i, on 

 the surface closely set with Coccoliths partly overlapping each other with their margins, partly leaving 

 corners uncovered between them. On an average ten Coccoliths may be placed on each Coccosphere. 

 When treated with diluted hydrochloric acid, an irregular, very insignificant mass of albuminoid matter 

 is left, which has kept no trace of the form of the Coccosphere; this form, therefore, must be pre- 

 served, either by the Coccoliths' being united with their edges, or rather by a system of radiating 

 calcareous bars in the middle; such bars, however, cannot be observed directly, the Coccoliths being 

 an absolute hindrance to the transparency of the Coccosphere; in a few instances it is possible, by 

 means of Nicholls placed crosswise, to see a faint, but rather regular black cross, showing that the 

 Coccosphere must be of a spherulitic structure; generally, however, the double refraction of the Coc- 

 coliths themselves prevents us from seeing this phenomenon. 



In polarised light between crossed Nicholls the Coccoliths show peculiar phenomena. The 

 marginal plate and the central sphere appear dark, and must consequently be composed of a 

 peculiar, single refracting form of carbonate of lime; but the central sphere shows a black cross, 

 which is not, as has else been mentioned, vertical, but oblique to different sides. The two annexed 

 figures show a Coccolith, seen in polarised light; to the right it is seen from the upper side, that is, 

 the side looking away from the Coccosphere, to the left from the under side. The figures are symme- 

 trical, both of them show a thinner, vertical beam, and a thicker transverse beam, the latter running 



