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 I/j'drol'ia jefih'jisi exisi'mg with 

 J/, ventrosa in the same ditches, but generally the MoUusca associated 

 with it have been fresh-water species, viz. : Bytliinia tentaculata, B. 

 leachii, Plafiorhis nautileus, P. spirobis, P. complauatus, and the 

 ubiquitous Lininaa 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 Limiicca truncatula ; 

 and I think it has at one time been accompanied by Assiminea 

 graya?ta, as I have taken numerous dead shells of the latter from its 

 habitat. 



A. grayana and Melampiis also seem to inhabit the same waters, 

 and upon one occasion I collected H. ulvce 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 

 venirosa, 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 Cireenwich Marshes, also wrote me to the 

 effect that at first suspecting them to be H. ventrosa he had sent them 



