NOTES ON THE MOLLUSCA OF THE THAMES ESTUARY. 221
animal and plant life of large tracts of marsh land, still we hope that
for years to come the shores of the estuary will afford ample scope
for collecting and observation. Within recent years the establish-
ment of various chemical, gas, and sewage works and factories has
caused several species of Mollusca to retreat some miles lower down
the river, and in one case, that of Hydrobia similis, will soon have
completely exterminated this rare and local form. As far as is at
present known, the only remaining British resort for this shell is a
small narrow ditch, a few yards only in length, the precise locality of
which, to prevent vandalism, it is necessary to keep a secret.
The conversion of the Thames into a gigantic sewer has almost
abolished "shrimping" near the mouth of the Thames, at Gravesend,
and the shoals of fish are fast retreating seaward. Last year a ditch
at Beckton, abounding with Hydrobia jenkinsi, and a very peculiar
tumid variety of the same shell, was completely poisoned by the
deposition of a quantity of chemical ballast. In the "Journal of
Conchology,'' vol. vi., page 141, Mr. J. T. Marshall, in an article on
the Hydrobiae and Assimineae from the Thames valley, mentioned the
fact, that "for some years Assiminea grayana has been migrating
down riverwards. Many years ago it was found abundantly in the
Greenwich marshes; but when Dr. Jeffreys, in 1868, wanted fresh
specimens for the purpose of illustrating his fifth volume, he could
find only two specimens, after a most diligent search," assisted by Mr.
Marshall; and his recorded habitat in that volume was "banks of the
Thames, between Greenwich and a little below Gravesend, making
altogether a distance of about twenty miles."
Mr. Marshall mentions that in his interleaved copy of Jeffreys',
the following note occurs, written in 1872 : "This habitat, which was
correct twenty years ago, has undergone some change in the interval.
At that time Clark and Barlee found it in myriads between Green-
wich and Charlton ; but at the present time neither Mr. Jeffreys nor
myself can find it there. We have, however, found it in countless
thousands at Abbey Wood, and Erith, on the raised banks of the
Thames, which now seems its nearest locality to London, so that they
appear to have migrated a distance of about ten miles ;" and Mr.
Marshall adds that "as Mr. Horsley has been searching for this
species also at the latter stations without success, it must have
migrated further still, if the sewage outfall works of recent years has
not altogether exterminated it."
A short time after this was written by Mr. Marshall, the Rev. J.