LOWER CHALK—MICROGRAPHIC STRUCTURE. 
They have been noticed bv many writers—by Ehrenberg, Huxley, 
Wallich, Sorby, and others ; and different opinions have been ex¬ 
pressed as to their nature and derivation. 
Recently, however, Messrs. Murray and Blackman* have investi¬ 
gated the characters of Coccoliths and Rhabdospheres, and have 
come to the conclusion that “ by far the larger number of bodies 
described by geologists as Coccoliths have no claim to be so con¬ 
sidered.” “ There is no evidence that they are derived from even 
allied organisms.” 
The result of their investigations is to show that Coccoliths pos¬ 
sess a well marked structure—viz., two limbs joined together by a 
central collar : “ The real test of a Coccolith is thus in its double 
structure.” They recognise two distinct forms which emanate 
from two species of Coccospheres. One is circular in outline with 
a small central pore, Goccosphcera leptopora; the other is oval with 
a central depression and a fairly large central aperture, sometimes 
cut into two D-shaped apertures by a cross bar of calcareous 
material, C. pelagica. The rims of both are usually ornamented 
by fine radiate striae. 
Examination of the finest dust from the Chalk Marl mounted, as 
recommended by these authors, in strong glycerine will show that, 
judged by the above standard, very few, if any, of the oval and round 
forms which abound in the deposit can be considered Coccoliths. The 
forms seen are oval, many have a central oval marking correspon¬ 
ding with the outline of the rim and are crossed by a central bar, 
in others the bar is absent. Viewed edgewise they appear like a 
watch-glass in shape, some are mere discs, but others have a greater 
thickness, and in such around the centre of the periphery may be 
seen a divisional line, but the true shirt-stud form was not detected. 
In many, however, the details of markings and structure are much 
obscured by calcitic crystals which adhere to them. It is, however, 
difficult to believe that at least some of these forms, whose ornamenta¬ 
tion and general appearance is remarkably like the true Coccolith, are 
not derived from organisms of a similar nature to the Coccosphere of 
to-day. Circular forms with a round central marking are also 
abundant. Rhabdoliths are not uncommon, and some of the 
circular forms appear to be the upper and funnel-shaped part of 
the Rhabdolith, the stalk being broken off. Messrs. Murray and 
Blackman have figured several specimens of “ spurious ” Coccoliths 
from the Chalk Marl, and also a Rhabdolith. 
(2) Crystals of calcite of irregular subangular, or rounded outline, 
clear and structureless, and differing in general appearance from 
those supposed to be shell-fragments. Such crystals are abundant 
in all washings, and must have been formed after the deposition 
of the chalky mud. The process by which many Chalk Marls 
* Murray and Blackman, Phil. Trans. Roy. Soc., Ser. B, Vol. cxc. p. 436. 
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