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 Assimineoe, which were originally 

 divided by I )r. 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 Hydrobiai for instance, of which family H. tilvcc 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 Physce. 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. venirosa 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 myosolis, and //. ulvie 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 //. u/vw at Lowestoft 



