68 NOTES AND MEMORANDA, 
full development. In specimens taken with the tow-net the 
spines are very usually absent; but that 1s probably on 
account of their extreme tenuity ; they are broken off by the 
slightest touch. In fresh examples from the surface, the dots 
indicating the origin of the lost spines may almost always be 
made out with a high power. ‘There are never spines on the 
Globigerine from the bottom, even in the shallowest water. 
Two or three very marked varieties of Globigerina occur; but 
I certainly do not think that the characters of any of them 
can be regarded as of specific value. 
The wonderfully pure calcareous formation in the neighbour- 
hood of Prince Edward Island and the Crozets consists almost 
solely of Globigerina bulloides. 
According to our present experience the deposit of Globi- 
gerina ooze is limited to water of a certain depth, the extreme 
limit of the pure characteristic formation being placed ata 
depth of somewhere about 2250 fathoms. Crossing from 
these shallow regions occupied by the ooze into deeper sound- 
ings, we find universally that the calcareous formation gra- 
dually passes into and is finally replaced by an extremely fine 
pure clay, which occupies, speaking generally, all depths 
below 2500 fathoms, and consists almost entirely of a silicate 
of the red oxide of iron and alumina. ‘The transition is very 
slow, and extends over several hundred fathoms of increasing 
depth; the shells gradually lose their sharpness of outline 
and assume a kind of “ rotten” look and a brownish colour, 
and become more and more mixed with a fine amorphous 
red-brown powder, which increases steadily in proportion 
until the lime has almost entirely disappeared. ‘This brown 
matter is in the finest possible state of subdivision, so fine 
that when, after sifting it to separate any organisms it might 
contain, we put it into jars to settle, it remained for days in 
suspension, giving the water very much the appearance and 
colour of chocolate." 
Orbulina.—There is still a good deal of obscurity about the 
nature of Orbulina universa, an organism which occurs in 
some places in large proportion in the Globigerina ooze. The 
shell of Orbulina is spherical, usually about °5 millimétre in 
diameter, but it is found of all smaller sizes. The texture of 
the mature shell resembles closely that of Globigerina, but it 
differs in some important particulars. The pores are markedly 
‘ «Preliminary Notes on the Nature of the Sea-bottom procured by the 
Soundings of H.M.S. Challenger during her Cruise in the Southern Sea 
in the early part of the year 1874.’ By Prof. C. Wyville Thomson, F.R.S., 
Director of the Civilian Scientific Staff on board. Read before the Royal 
Society, Nov. 26, 1874. 
