a ae ee 
2F 
assnme that the sea was originally perfectly fresh. The chief error is 
probably due to sea areas being detached from the main ocean. 
The most probable figure for the time since the globe was cool 
enough for water to condense is 90,000,000 years. 
MICROSCOPICAL SECTION. 
The only meeting of this Section during the Session took place on 
Friday, December 15th, and was well attended by the members. Ex- 
ceptional interest was taken in the exhibit by Mr, J. D. Siddall, who 
showed, with the aid of the newly-invented dark-ground illuminator for 
high powers, a new marine diatom (Coscinodiscus heliozoides), which he 
found in a gathering sent to him by Mr. Thos. Shepheard, of Bourne- 
mouth. 
The pseudopodial extensions of the protoplasmic contents of the 
diatom were most remarkable, and open out a probable solution of the 
very controversial nature of the movements of these minute organisms. 
Photographs have been taken, and a fully desvriptive Paper has been 
read by Mr. Siddall before the Royal Microscopical Society, and Mr. 
Siddall gives the following brief account of the discovery :— 
Notes on Coscinodiscus heliozoides, n.sp., Siddall. 
In a surface-netting sent to me early in December last trom 
Bournemouth by Mr. Shepheard, I found a number of small discoid 
living Diatoms, apparently of the genus Coscinodiscus, but each fringed 
with a double series of outspread radiating filaments so exceedingly 
delicate and pellucid as only to be visible when illuminated with a high- 
power black-ground condenser. ‘This unique observation I communicated 
at once to Mr. Shepheard and Mr. Waddington, both of whom obtained 
many further nettings, and kept me fully supplied with living material 
for further study until the end of May last, after which date the diatoms 
disappeared. 
I have endeavoured, by careful examination and study of living and 
mounted specimens. to make ont as much as possible of the structure 
and life-history of this particularly interesti:g Diatum, and very briefly 
epitomise the results. 
In size (zjp in diameter) and marking (hexagonally arranged 
primary and secondary dots, and narrow striated border pierced by 
canals) it most closely resembles Coscinodiscus radiatus, but differs by 
possessing these radiating filaments. Numerous chromoplasts give it 
a yellow-brown colour; these are embedded in colourless, slightly granu- 
lated protoplasm. The protoplasm has a central nucleated mass, from 
which radiating strands extend to the border, and enter and protrude 
through the canals. The whole outer surface of the Diatom is also 
coated with protoplasm. Watery fluid fills the interstices between the 
strands of protoplasm. From 40 to 50 long slender unbranched filaments 
radiate from each border of the frustule. ‘hese originate as extensions 
through the border canals of the cell plasm. From a slender, barely 
discernible, flexible thread they elongate, thicken, and stiffen, and when 
tully developed vary in length from one to three or four times the 
diameter of the disc. The superficial protoplasm also coats the filaments, 
and gives them a slightly granular appearance. So coated, each becomes 
a tactile organ capable of lifting and moving the Diatom about. In 
the mature frustule the filaments all disappear. They also disappear 
entirely and are dissolved by cold nitric acid, so that the silicification (?) 
of the axis of each is very slight. 
I have watched these living Diatoms for hours in a trough, and 
have distinctly seen the filaments change position and move the Diatom 
by so doing. Therefore, I maintain that they are locomotive organs, 
