222 NOTES ON THE MOLLUSCA OF THE THAMES ESTUARY,
W. Horsley obtained a number of this species from the river bank at
Gravesend : and subsequently we collected together a number of
living Assimineae with Melampus myosotis at the Salt Marsh near
Purfleet. I have frequently taken dead shells from the same habitat
as Hydrobia similis, and also below Erith ; but I have never found
living specimens above Greenhithe and the Purfleet Salt-marshes.
The marshes between Greenwich and Woolwich were for some
years the recorded habitat of Hydrobia similis ; but it has long since
disappeared from that locality, together with the original colony of the
new Hydrobia which I discovered in East Greenwich Marshes in
1883.
It thus appears that in a period of about a quarter of a century,
several species have been forced to migrate lower down the river from
the causes above mentioned. The same causes will also account for
the total extinction of rare or local forms not sufficiently vigorous to
reproduce their species quickly, or to adapt themselves to new habits
and environment.
The list of Mollusca appended to this paper is the faithful record
of two years' work ; but, as I have said, it is by no means complete,
and I am certain that members of the Essex Field Club in collecting
along the Essex Marshes will be able to add to it considerably.
Of shells peculiar to the marshes, eighteen fresh-water species, six
brackish-water, and nineteen species of land shells have been recorded;
or a total of forty-three species. Adding those collected in the lanes
and hedgerows in close proximity brings the number up to fifty-four
species, with thirty-two varieties.
It is to be regretted that circumstances have not at present per-
mitted a study of the mouth of the Thames for marine forms : neither
has there been time for collecting land shells upon the Essex Marshes.
But this last omission is less to be regretted ; for, on reading the very
interesting account of the "Land and Fresh-water Mollusca of Wan-
stead and the neighbouring districts of the Becontree Hundred," by
Mr. Crouch in vol. iv. of the Essex Naturalist, I came to the con-
clusion that in all probability the Terrestrial Mollusca that are most
common upon the Kentish Bank, are also likely to be the prevail-
ing species inhabiting the Essex Marshes.2
I have little to say respecting the land and fresh-water shells of
this district ; but will take the opportunity of making some remarks
2 I was also pleased to find that the pretty little many-whorled shell Planorbis contortus, which
I had reason to believe was both rare and local, having only taken four shells once in two years,
has been recorded by Mr. Crouch as common in quite a number of localities in the Becontree
Hundred.