52 
LENHAM BEDS. 
there is a marked unconformity at the base of the Pliocene. In 
Belgium the most recent deposits found beneath the Pliocene are 
the Miocene beds known as Bolderian and Anversian/^' 
We have now traced the sandy, or Diestian, type of Lower 
Pliocene from Kent through northern France to Belgium, but it 
still remains to compare the faunas of the different districts. 
This may fairly be done, for lithologically the beds are similar; 
while the bathimetrical depths at which the shells lived seem to 
have been nearly the same, and the mollusca must also have 
inhabited closely adjoining portions of the same sea, with no 
barrier between them. Under these circumstances we have a 
right to expect a close resemblance between the faunas, if they 
are truly contemporaneous. 
Before analyzing the lists it may be well to point out any slight 
local circumstances which may have modified the character of the 
fauna, and which should be allowed for when we compare the 
different areas. The shells found at Lenham come from a bed 
that lies almost directly upon the flinty Chalk. They include 
abundant Arcadce, and numerous rock-loving gasteropods, mixed 
with the species commonly found in sand, but no truly littoral 
forms, no RissocE, and few herbivorous gasteropods of any sort, 
though the carnivorous species are abundant. The Chalk 
probably formed a bare submerged reef for part of the time. 
The depth of water over this reef must have been considerable, 
for most of the bivalves have their valves united, broken shells 
are rare, and worn specimens are unknown. Taking everything 
into consideration, I think we cannot place the depth of water 
at less than 40 fathoms. 
In the area around Diest, where the sands rest on a great 
variety of beds, but the fauna is that found on a sandy bottom, 
there is no evidence that reefs of the older rocks were bare for 
a sufficient length of time to influence the character of the fauna. 
Though the beds around Diest occur at a considerably lower 
level than the top of the Downs near Lenham, they contain 
broken shells more abundantly, and the fauna has perhaps a 
somewhat more littoral facies. 
At Antwerp the sand is replete with the fauna belonging to 
the deposit in which the shells are now found, without any 
mixture of rock-loving forms. Though the locality lies 600 feet 
lower than Lenham, the depth of water seems to liave been 
about the same, perhaps slightly less. 
In a deep well at Utrecht, Diestian beds were found to extend 
to a depth of at least 1,140 feet beneath the sea.f Yet the 
character of the fauna—as far as one can judge by the specimens 
* See also P. Cogels, Considerations nouvelles sur les systemes bolderien et 
diestien. Ann. Soc. Malacol. de Belgique, vol. xii. pp. 7-26, (1877,) and E. Van den 
Eroeck, Decouverte de Eossiles miocenes dans les Depots de I’Etage Bolderien. 
Ibid., vol. xix. pp. Ixviii.-lxxv. (1884.) 
f .L Lorie, Contributions a la Geologic des Pays>Bas. I. Kesultats geologiques 
et paleontologiques des Forages de Puits a Utrecht, Goes et Gorkum. Archives 
du Musee Teyler. Ser. II., vol. ii., 3rd part. (1885.) 
