LOWER CHALK—DEPTH OF THE SEA* 
357 
of genera which are absent from or are very rare in the former, 
such are Astarte , Corbis, Cardium, Crassatella , Cyprina, Gastro- 
chcena, Pectunculus, Trigonia , and Bolen (? Solecurtus) : genera 
which all now live in shallow waters, and might well be found 
together in about 50 fathoms. There can be little doubt that the 
water was much shallower in Devon than it was in Kent. 
From the Brachiopoda there is little to be learned, except that 
the conditions of the Lower Chalk were much more favourable 
to them than those of the Gault. From the Lower Gault of Folke¬ 
stone Mr. F. G. Price records only the doubtful occurrence of one 
species. In the Upper Gault there are two genera and two species ; 
in the Chalk Marl he found four genera and nine species. Put this 
increase is probably attributable to the fact that these genera dis¬ 
liked muddy water then, as they do now, though they can live on 
clean sand, for out of twenty species found in the Chalk Marl 
generally fourteen occur in the Warminster beds. 
Of Echinodermata there is a similar increase in number both 
of species and of individuals in passing upward to the Chalk Marl 
in the south-east of England, as shown in the following table : — 
Genera. Species. 
In the Lower Gault - -- -.-3 4 
In the Upper Gault ----- 5 8 
In the Chalk Marl ------ 11 26 
This increase maybe partly attributable to the same cause, but as 
only fourteen species range up from the Warminster beds into 
the Chalk Marl of Wiltshire and the Isle of Wight, some of those 
in the latter may have come in with deeper water. 
Foraminifera. — These minute organisms probably furnish us 
with better bathymetric evidence than any other class, because 
so many of the Cretaceous species have survived without change 
to the present day, and because the range and distribution of the 
recent species is now fairly well known. 
The Foraminifera found in a deep-water oceanic deposit differ 
from those found in the lesser depths nearer to land in the larger 
proportion of pelagic species and in the smaller number of species 
that live on the bottom. Shallow-water deposits on the other hand 
generally contain a number of arenaceous forms, whose tests are 
formed largely or entirely of minute grains of sand. 
The abundance of such Arenaceous Foraminifera in the marly chalk 
of the Am. varians zone, their scarcity in the higher beds, and the 
significance of these facts was first pointed out by Dr. A. F. Hume* ; 
the significance being that at the present day the coarser arena¬ 
ceous Foraminifera are abundant down to the depth of 400 fathoms, 
but beyond that depth they -rapidly become scarce, only a few of 
the more widely distributed species being found at greater depths. 
He also indicated the remarkable resemblance that exists between 
* Chemical and Micro-Mineralogical Researches on the Upper Cretaceous 
Zones, p. 81, London, 1893. See also Natural Science. Vol. vii., p. 270 
(1895). 
4219. 
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