196 SOME MINOR PROBLEMS CONCERNED IN THE LOCAL
list which have passed, or are passing away, we can assign absolutely
no reason, nor does it seem that we are on the road to a solution ;
their food even, for the most part, is not known.
Some forms, on the other hand, now living in fair numbers, but
not represented in the marl, seem to claim to be regarded as new
introductions. It is true that negative evidence is always suspicious,
but it is also true that the marls have been well examined. There is
no reason that we know for the preservation of one shell more than
another; therefore, if they had lived on the spot, we might reasonably
suppose that their remains would have been found. The largest of
these apparently new introductions is Helix virgata. In my own
parish (Felstead) it is rare. In an adjoining parish (Stebbing) it is in
one part very common, occurring literally by thousands. It is
quoted by Mr. Crouch (E. N., vol. iv., page 210) as occurring near
Barking Creek. It is also included in Dr. Laver's list. (Trans.
E. F. C, vol. ii., p. 95). Its absence from the marls is very striking,
as it is a conspicuous shell, and gives us every reason for believing
it to be a new arrival.
I may here add that I am continually expecting the arrival of
four other large forms which, on the same reasoning, have never
before visited this neighbourhood. They are the two Paludinas,
Unio tumidus, and Planorbis corneus. The first is common in South
Essex and surrounding counties, the second in many parts of Essex
and surroundings, and the last, long a resident in South Essex,
occurring fossil in the Thames alluvium, has now reached Chelms-
ford, as we have seen.
Our local list of reptiles furnishes but little from our present
point of view. The case of the Common Ringed Snake and the
Adder demands a passing notice. From what information I can
obtain, which is confessedly fragmentary, they both appear to be
dying out in a tract of land say ten miles wide, beginning at
Danbury Common and extending northwards as far as the river
Stour. Thus, in the Sampfords and Bardfields, both species appear
to be extinct. At Finchingfield and Wethersfield and Stebbing,
snakes are very rare and adders are unknown. In the neighbourhood
of Felstead both have markedly diminished within the last forty
years, whilst at Danbury Common both species still survive in
numbers, although the snakes are much in excess of the adders. I
have taken much trouble to obtain information as to my own neigh-
bourhood, and believe I am correct in saying that the adder is now