222 NOTES ON THE MOIT.USCA OF THE THAMES ESTUARY, 



W. Horsley obtained a number of this species from the river bank at 

 Gravesend : and subsequently we collected together a number of 

 living Assimineas with Melampus myosotis at the Salt Marsh near 

 Purfleet. I have frequently taken dead shells from the same habitat 

 as Hydrobia similis, and also below Erith ; but I have never' found 

 living specimens above Greenhithe and the Purfleet Salt-marshes. 



The marshes between Greenwich and Woolwich were for some 

 years the recorded habitat of Hydrobia similis ; but it has long since 

 disappeared from that locality, together with the original colony of the 

 new Hydrobia which I discovered in East Greenwich Marshes in 

 1883. 



It thus appears that in a period of about a quarter of a century, 

 several species have been forced to migrate lower down the river from 

 the causes above mentioned. The same causes will also account for 

 the total extinction of rare or local forms not sufficiently vigorous to 

 reproduce their species quickly, or to adapt themselves to new habits 

 and environment. 



The list of MoUusca appended to this paper is the faithful record 

 of two years' work ; but, as I have said, it is by no means complete, 

 and I am certain that members of the Essex Field Club in collecting 

 along the Essex Marshes will be able to add to it considerably. 



Of shells peculiar to the marshes, eighteen fresh-water species, six 

 brackish-water, and nineteen species of land shells have been recorded ; 

 or a total of forty-three species. Adding those collected in the lanes 

 and hedgerows in close proximity brings the number up to fifty-four 

 species, with thirty-two varieties. 



It is to be regretted that circumstances have not at present per- 

 mitted a study of the mouth of the Thames for marine forms ; neither 

 has there been time for collecting land shells upon the Essex Marshes. 

 But this last omission is less to be regretted ; for, on reading the very 

 interesting account of the "Land and Fresh-water Mollusca of Wan- 

 stead and the neighbouring districts of the Becontree Hundred," by 

 Mr. Crouch in vol. iv. of the Essex Naturalist, I came to the con- 

 clusion that in all probability the Terrestrial Mollusca that are most 

 common upon the Kentish Bank, are also likely to be the prevail- 

 ing species inhabiting the Essex Marshes." 



I have little to say respecting the land and fresh-water shells of 

 this district ; but will take the opportunity of making some remarks 



2 I was also pleased to find that the pretty little inany-whorled shell PlanorHs contortus, which 

 1 had reason to believe was both rare and local, having only taken four shells once in two years, 

 has been recorded Ijy Mr. Crouch as common in quite a number of localities in the Becontree 

 Hundred. 



