WITH A LIST OF SPECIES OBSERVED. 223
with regard to the brackish-water species, and also a few words of
explanation of the difficulties which have arisen, by reason of the
discovery of the new Hydrobia, which is now generally considered by
conchologists as worthy of specific rank.
Our British Hydrobiae and Assimineae, which were originally
divided by Dr. Jeffreys into two distinct orders, would seem to occupy
a somewhat anomalous place in the Molluscan world. They are not
all strictly marine in habit, neither can they all endure long immer-
sion in fresh water. To me they appear to form distinctly a con-
necting link between the fresh-water and the marine Mollusca.
Take the Hydrobiae for instance, of which family H. ulvae nearest
approaches to the purely marine species ; whilst jenkinsi, by the
readiness with which it adapts itself to fresh-water environment
(during which it will remain hardy and vigorous for prolonged
periods, reproducing its species with remarkable fertility), certainly
serves to connect the two groups. H similis also does not object to
water that is not in the least brackish ; indeed, specimens remained
alive in tap water in Aquaria after many months (but I could not
succeed in breeding them under artificial conditions). Dr. Jeffreys
states in his "British Conchology" that in France they inhabit quite
fresh water. Under these circumstances it is rather difficult to
properly classify this family. They can scarcely be all designated as
fresh-water Mollusca, neither does it seem quite right to include them
all with the marine shells; possibly the best way out of the difficulty
would be to constitute them an intermediate class.
Hydrobia ventrosa has a persistent habit of floating shell down-
wards upon the surface of the water after the manner of the fresh-
water Physae. H. jenkinsi and H. similis may occasionally have
recourse to the same habit, but not habitually. The two latter
species will also crawl about immediately after being placed in a saucer
without sufficient water to cover them, and the former will extend its
researches over the edge of the dish and even upon the table.
H. ventrosa is very timid if disturbed in this way ; it will remain
quite dormant for a long time, and it never travels out of the reach
of water.
A. grayana, Melampus myosotis, and H. ulvae are great wan-
derers also, and may frequently be collected many yards away from
the water, or high and dry upon mud flats, and crawling upon
wooden piles some distance from tide mark. In such a situation
last summer I collected some thousands of H. ulvae at Lowestoft