1890-91.] Dr Murray and Mr Irvine on Silica in Seas. 
233 
from neritic* Plankton, which latter term includes surface organisms 
in waters near coninental and other coasts. f They would appear, 
however, on the whole, to prefer oceanic waters where the salinity of 
the water is relatively low, for in the very salt waters of the Red Sea, 
of the Mediterranean, and of the trade-wind regions of the Atlantic, 
although numerous, they are not apparently so abundant as in the 
less salt waters of the Pacific, Indian, Southern, or Polar Oceans. 
The tow-net experiments of the “Challenger” Expedition showed that 
those Radiolarians which secrete the heaviest shells and skeletons, 
as well as the whole legion of Phseoclaria, were captured in greatest 
numbers when the nets were dragged a considerable distance beneath 
the surface, so that it is probable that many species live in the inter- 
mediate waters of the ocean where the temperature is as low as 
50° or 40° E. Some species of Diatoms and Radiolarians are often 
met with in such great numbers that they form vast floating banks, 
fields, or zones, between which are lanes of water comparatively free 
from these organisms. We have referred to the banks of Diatoms 
in the Arctic and Antarctic Oceans and in the Arafura Sea. In 
1880, in the Faroe Channel, the tow-nets were filled with Radio- 
larians, belonging to the genera: Acanthometra , Xiphacantha , 
Dorataspis, Etlimosplicera , Heliosphcera , Rliizospkcera , Actinomma , 
Spongocyrtis, Tlialassicolla , Calcaro?nma, Actinocyrtis, Amphilonche , 
Spongodiscus , and Thalassosphcera — for weeks together; but two 
years later, in the same month, only a few of these organisms were 
captured, the surface waters being then chiefly occupied by vast 
numbers of Doliolum. 
If we now turn to the remains of Diatoms and Radiolarians found 
in the marine deposits at the bottom of the ocean, we find that they 
are almost universally distributed. Radiolarian remains were ob- 
served in considerable abundance in more than two-thirds, and 
Diatoms in like abundance in more than one-half, of the samples of 
deep-sea deposits collected by the “ Challenger,” and a careful exami- 
nation of large samples revealed the presence of these organisms in 
nearly every specimen of pelagic and terrigenous deposits. In the 
deep-sea deposits of the Atlantic, Radiolarians, Diatoms, and Sponge 
spicules, make up, on an average, about 1J- per cent., in the Pacific 
* Nripirris, son of Nereus (see Haeckel, Plankton- Studien, p. 22, Jena, 1890). 
t Haeckel, loc. cit. 
