494 THE DEPTHS OF THE SEA. [cnMAP. X. 
horny substance enclosing the greater part of the 
curved spiral shell. Now if the recent Spirula had 
been weighted with such a rostrum it would probably 
have remained up to the present time utterly unknown 
to us. It is unwise to prophesy, but I certainly 
look upon some form allied to Spirulirostris as one 
of the most likely spoils of the deep sea. From the 
Tertiaries we pass to the Cretaceous forms, and find 
in Belemnitella the chambered shell straightened and 
reduced, and the ‘guard’ greatly increased in size. 
If Belemnites were deep-sea animals, as seems very 
probable, and if any of them still exist,—-from the 
form and weight of their shells it is scarcely possible 
that they should ever be thrown up on the shore, 
and without deep-sea dredging they might remain 
for ever unknown. I merely mention this to show 
that it is by no means safe to base even what little 
argument might rest upon it, upon the absence 
at the present time of all representatives of the 
cretaceous cephalopodous fauna. 
The gasteropods, with comparatively few excep- 
tions, range from the shore to a depth of 100 to 
200 fathoms, and lamellibranchs become scarcer at 
a slightly greater depth; while some orders of bra- 
chiopods, crustacea, echinoderms, sponges, and fora- 
minifera, descend in scarcely diminished numbers to 
a depth of 10,000 feet. In fact, the bathymetrical 
range of the various groups in modern seas corre- 
sponds remarkably with their vertical range in 
ancient strata. 
A change in the distribution of sea and land in- - 
volving a mere change in the course of an ocean- 
current might modify the conditions of an area for 
