224 NOTES ON THE MOLLUSCA OF THE THAMES ESTUARY,
by just brushing them into a box from off the piles of a landing
stage.
Upon two occasions I have found Hydrobia jenkinsi existing with
H ventrosa in the same ditches, but generally the Mollusca associated
with it have— been fresh-water species, viz.: Bythinia tentaculata, B.
leachii, Planorbis nautileus, P. spirobis, P. complanatus, and the
ubiquitous Limnaea peregra. Once I found a single living H. similis
with H. jenkinsi in a new locality upon Erith Marshes.
I have found Hydrobia similis associated with Limnaea truncatula ;
and I think it has at one time been accompanied by Assiminea
grayana, as I have taken numerous dead shells of the latter from its
habitat.
A. grayana and Melampas also seem to inhabit the same waters,
and upon one occasion I collected H. ulvae from the same ditch
upon Dartford Marshes, in which these two species were abundant.
A curious dwarfed variety of Littorina rudis occurs in the brackish-
water ditches upon West Tilbury Marshes along with Hydrobia
ventrosa, and the latter species and H. similis are also found together.
I believe that H jenkinsi is the most abundant Thames marsh
species of the Hydrobiae, and its habitat extends far beyond the
others, occupying many miles of ditches from the commencement of
the Plumstead Marshes, near the Arsenal wall, away down to a point
midway between Dartford Creek and Greenhithe, and from Beckton
nearly to Coldharbour Point, which to me appears to be the full
extent of its distribution in Essex. I made my first acquaintance
with this interesting Mollusc during the early summer of 1883, when
I collected from a muddy ditch upon the marshes near East Green-
wich six or eight specimens of a small operculated Mollusc, which
did not agree with any British shell with which I was at that time
acquainted.
The animal seemed to me to differ entirely from the genus
Bythinia, and the operculum, in particular, was quite distinct, and
seemed to more nearly approach that of the Littorinidae. I made a
drawing of the animal and its shell, and sent off by the post a
number of specimens to several conchologists of my acquaintance,
and they were unanimous in pronouncing them to be Hydrobia
similis, Drap.
Another well-known conchologist to whom I sent specimens of
the same shells from East Greenwich Marshes, also wrote me to the
effect that at first suspecting them to be H ventrosa he had sent them