CHAP. x. | CONTINUITY OF THE CHALK. 493 
the shallower is the water which it inhabits. The 
cephaiopods are chiefly pelagic and surface things, 
and their remains are consequently found in deposits 
from all depths. To this general pelagic distri- 
bution of cephalopods there seem to be two re- 
markable exceptions, and these the two members 
of their class which are by far the most interest- 
ing in their geological relations. Nautilus pom- 
puius inhabits the deep water of the Pacific, while 
the habitat of Spirula australis is unknown. The 
shell of Spirula is thin and light, and, probably 
after the death of the animal and by the decom- 
position of organic matter, it becomes filled with air, 
and the emptied shell floats, and is drifted along on 
the surface of the sea. Tropical shores are strewn with 
the pearly little coil, which attracts attention by the 
elegance of its form. It is abundant on all shores in 
the path of the Gulf-stream. Sysselmann Miiller gave 
me, afew years ago, a quantity which had been drifted 
on the south-western shores of different islands of the 
Feéroe group. Still the structure of the animal of 
Spirula may be said to be unknown. One specimen 
only, which was described by Professor Owen, was 
found nearly perfect on the coast of New Zealand by 
Mr. Percy Noel. I suppose there can be little doubt 
that this is a deep-water form, and I hope that with 
our deep-sea dredging we shall soon clear up its 
economy ; but inthe meantime the evident abundance 
of the animal and our ignorance of its history are very 
suggestive. In the London clay one or two examples 
of a fossil have been found, nearly allied to Spirula, 
but differing in this respect—that a solid conical 
rostrum projects backwards, its half-calcified, half- 
