26 
Cephalopoda are found to a depth of 12,000 feet; Echinoidea to 
17,000 feet; the Crinoida to a depth of 17,000 feet ; the Brachio- 
poda to 17,000 feet; the Gasteropoda, in very diminishing 
numbers and variety, to 15,000 feet. Fish are found at depths 
of 17,000 feet at twenty observation stations. 
The lesson derived from the examination of the conditions 
of Life in the Marine Zones is the same as in the Terrestrial, 
adaptation to environment or extinction. 
The Fossil Zones of the Chalk in Brighton and 
Neighbourhood. 
From the consideration of the elementary and _ well- 
known zones of existing life, we now come to the less known 
zones of fossil life. The conditions are widely different in the 
fossil state. We now depart from the contemporaneous view, 
and have to examine the life history of genera and species, 
deposited in the chalk, at vast intervals of time, the greater part 
of them appeared for the first time, and became extinct with the 
close of the deposition of the chalk. I have endeavoured to show 
in the existing zones of contemporaneous life, an adjustment to 
their terrestrial and marine environments, The marine survey 
was over a wide area of the existing seas, and a smaller extent of 
land. The area of the cretaceous sea was very great. I shall be 
able later to exhibit a co-relation of fossils deposited in nearly 
similar conditions at a distance of 6,000 miles apart. The 
cretaceous sea was perhaps not inferior in area to the Atlantic or 
the Pacific Oceans,—certainly larger than the Mediterranean. 
The exact area is impossible to be ascertained; denudation has 
removed the evidence of its deposition over many parts of 
England, and nearly al: of Scotland. Tertiary volcanoes have 
probably largely metam orphosed and destroyed the chalk in the 
Island of Mull and Scotland generally, where the remains of the 
cretaceous sea are isolated and scanty. The cretaceous sea, like 
the modern ocean, had its shores and depths, for it is a well- 
recognised opinion that, since the beginning of geological time, 
there has been a division of the globe into sea and land, and 
consequently there were various degrees of depths and currents, 
as we find in the present oceans. The chalk is largely composed 
of microscopic foraminifera and the more or less minute frag- 
ments of the shells of mollusca. The proportion varies greatly : 
generally, the deeper the water the more minute the fragments 
and the purer white the colour of the chalk. In interpreting the 
history of the Fossil Zones, we have not the same absolute proof 
of their position that we had in the Existing Zones ; we have to 
rely largely on the paleontological evidence, supported in a 
lesser degree by the lithological, The exposures of the chalk in 
Brighton and district are as complete as in any other County in 
