Proceedings. XXVli 
which the animal swims actively. The shell is generally cal- 
careous, and very delicate in texture, frequently taking the form 
of a slender hollow cone. Pteropod ooze is not found below 
1500 fathoms, the carbonic acid in the water dissolving the 
delicate shells. For the same reason Globigerina ooze dis- 
appears below 2500 fathoms, the shell being somewhat thicker. 
Sponges. —The simplest form of sponge consists of a thin- 
walled sac with the walls perforated by pores, and the central 
cavity lined by flagellated cells. The constant vibration of 
these flagellated cells gives rise to a strong current of water, 
which is drawn through the pores into the central cavity, and 
passes out through the mouth of the sac. About 2000 species 
are known, and 800 are found round the British coast. They 
may be either soft, stony, leathery, or horny, and vary in weight 
from one grain to over one hundred pounds. The more com- 
plicated forms are built up of a number of cells supported on a 
framework either of horny fibres or of siliceous or calcareous 
spicules. The skeletons of the siliceous species form extensive 
deposits in deep water. They are divided into four orders :— 
(1) Monaxonida, with one-rayed spicules; (2) Tetractinellida, 
with spicules of four rays ; (3) Lithistida, which are massive and 
stony sponges, with interlocking spicules; (4) Hexactinellida, 
with six-rayed spicules. The calcareous and horny sponges 
form separate groups—the Calcarea and Keratosa. 
Diatoms are one-celled plants, belonging to the Algx, in- 
habiting both fresh and sea water. The cell-wall is hardened by 
the deposition of silica, so as to give rise to a glassy case, known 
as the frustule, composed of two parts which fit into one another 
like the lid of a pill-box. The cells may be either separate or 
connected into filaments, either free or attached to some other 
body. The separate valves are of various forms, and their sur- 
faces exhibit more or less delicate sculpturings and markings, or 
they present a cellular appearance like honeycomb. The accumu- 
lation of these flinty envelopes gives rise to very extensive de- 
posits, more especially in the Arctic and Antarctic regions, the 
total of which has been estimated at 10,880,000 square miles. 
Minute bodies known as Coccoliths and Rhabdoliths are to 
be met with in nearly all deep-sea deposits. Coccoliths are oval 
caleareous discs, having a thick strongly refracting rim and 
centre, and are the disintegrated remains of a spherical body 
known as a coccosphere. Rhabdoliths consist of minute calcareous 
rods, having a disc at one end, and are the disintegrated remains 
of a body called a rhabdosphere. Coccospheres and rhabdo- 
spheres are now regarded as pelagic Alge. 
From a depth of 2000 fathoms downwards there is a wide- 
spread deposit of red and grey clays, the area of which is esti- 
mated at 51,500,000 square miles. These consist of silicate of 
