64 
ST. ERTH BEDS. 
As there appears to be a considerable number of undescrlbed 
species, besides the MS. names given in the above list, it is 
difficult to arrive at the relative proportion of living and extinct 
forms contained in the deposit. Messrs. Kendall and Bell men¬ 
tion about 20 more that do not seem to be known, either in a 
fossil or recent state, making a total of 92 species and marked 
varieties of mollusca discovered up to the time their paper was 
written. Of these about 38, or 41 per cent., appear to be extinct, 
a proportion exactly corresponding with that found in the 
Coralline Crag. Probably when the St. Erth fauna is better 
known the proportion of extinct species will be found to be 
greater, for species dying out will commonly be represented by 
fewer individuals than species with a wide range in time. As far 
as the per-centage test is of value, and I do not like to trust 
very much to it, it appears to show that the St. Erth clays are 
fully as old as the Coralline Crag. 
Another test of the age of these newer Tertiary deposits is 
afforded by a consideration of the climatic conditions indicated 
by their fauna. The evidence from this source likewise indicates 
that the St. Erth Clay is Older Pliocene, not Newer Pliocene. 
In the last chapter we traced the gradual change of temperature 
which is revealed by the Older Pliocene beds as they are followed 
through Holland, Belgium, and Suffolk, to Kent, that is to say 
from north-east to south-west. We now find that the St. Erth 
clays yield a fauna such as might have lived in warm seas like 
those wherein the sands of Lenham were deposited. They are 
marked by a similar enormous preponderance of southern mollusca; 
at the same time the species found at the two localities are 
different. 
I will now give the reasons which have led me to consider 
that, notwithstanding the character of the included fauna, the 
St. Erth clays were deposited in so great a depth as 40 or 50 
fathoms. This is the more necessary as a glance at the list of 
species, or even an examination of a collection of the fossils, will 
seem at first strongly to confirm the justice of Mr. Belfs criticism, 
that most of the species belong to the Laminarian zone, that is to 
say, that they did not live in a greater depth than about 15 
fathoms. The list by itself clearly points to that conclusion. 
We have, however, to deal with a much more complicated 
question, for firstly we must inquire under what physical con¬ 
ditions the beds could have been deposited; secondly, we must 
take into account the close proximity of shoaler water; and 
thirdly, we ought to separate the species which lived on the spot 
from those that belonged to the shoaler water, and were merely 
washed or dropped into this basin. 
First with regard to the question of the physical conditions 
which prevailed, it has already been pointed out that the clay lies 
in a pass or valley dividing two tracts of high land. The fossi- 
liferous strata occur at almost exactly 100 feet above the present 
sea-level, though unfossiliferous sands, probably belonging to 
