WITH A LIST OF SPECIES OBSERVED. 227
mistaken for it. Professor E. von Marten has also noticed this,
although he believes that it differs from jenkinsi in other respects.
It seems extremely probable that this new species has been
introduced from abroad, particularly as it has not been noticed in
any other part of Great Britain. From the fact that none of our
other Hydrobiae seem to have the least tendency to assume the
keeled and tufted appearance, surely, if indigenous, it must have
been noticed long ago. It seems hardly possible that Dr. Jeffreys,
Mr. Marshall, and other conchologists of repute, could have over-
looked the species. But, supposing it to be a recent introduction, we
are met with the further difficulty that it is almost inconceivable that
the Hydrobia could have increased to such an extent in a few years.
The shells now occupy many miles of ditches upon both sides of the
Thames, and from their greater vigour and prolifieness they seem
likely before long to predominate over our other native species.
The carinated tufted specimens of H. jenkinsi are so distinct, and
generally they so far exceed the proportion of shells which are not
keeled or tufted, that I prefer to consider them as typical of the
species ; those specimens which are without any trace of either keel
or tufts may be called var. ecarinata. Besides these two forms, there
are to be found existing with the type at Beckton, and several other
places, a very short spired, much inflated variety, which I have pro-
posed to name var. tumida. Other specimens occur at Beckton and
Plumstead which are much more graceful in contour, the spire is long
and tapering, and the outer lip very much expanded ; if worthy of
being considered as a variety, they might be designated var. gracilis.
There is every reason to believe that the pretty little shell H.
similis (or H. confusa, Frau., as they prefer to call it at the Natural
History Museum at Kensington), will not long continue to be
enumerated with the British Hydrobiae. A recent visit to its habitat
resulted in obtaining two dead shells only, and the most diligent
search for living specimens was unavailing. It is just possible that a
few may still remain in hybernation, as the locality has been known to
but a few conchologists, who have done their best to preserve it from
extinction.
In searching for this Mollusc some very interesting caddis-worm
(Phryganidae) cases have at times been collected, which have been
constructed almost entirely of the young shells of H. similis and of
H. ventrosa; and upon one occasion I came across a caddis-case
which had a single shell of the subterranean species Coecilioides