NOTES ON THE MOI.LUSCA OF THE THAMES ESTUARY. 22 1 



animal and plant life of large tracts of marsh land, still we hope that 

 for years to come the shores of the estuary will afford ample scope 

 for collecting and observation. Within recent years the establish- 

 ment of various chemical, gas, and sewage works and factories has 

 caused several species of Mollusca to retreat some miles lower down 

 the river, and in one case, that of Hydrobia similis, will soon have 

 completely exterminated this rare and local form. As far as is at 

 present known, the only remaining British resort for this shell is a 

 small narrow ditch, a few yards only in length, the precise locality of 

 which, to prevent vandalism, it is necessary to keep a secret. 



The conversion of the Thames into a gigantic sewer has almost 

 abolished " shrimping " near the mouth of the Thames, at Gravesend, 

 and the shoals of fish are fast retreating seaward. Last year a ditch 

 at Beckton, abounding with Hydrobia Jeukinsi, and a very peculiar 

 tumid variety of the same shell, was completely poisoned by the 

 deposition of a quantity of chemical ballast. In the " Journal of 

 Conchology," vol. vi., page 141, Mr. J. T. Marshall, in an article on 

 the Hydrobice and AssifninecE from the Thames valley, mentioned the 

 fact, that " for some years Assi?fn?iea grayana has been migrating 

 down riverwards. Many years ago it was found abundantly in the 

 Greenwich marshes ; but when Dr. Jeffreys, in 1868, wanted fresh 

 specimens for the purpose of illustrating his fifth volume, he could 

 find only two specimens, after a most diligent search," assisted by Mr. 

 Marshall ; and his recorded habitat in that volume was " banks of the 

 Thames, between Greenwich and a little below Gravesend, making 

 altogether a distance of about twenty miles." 



Mr. Marshall mentions that in his interleaved copy of Jeffreys', 

 the following note occurs, written in 1872 : " This habitat, which was 

 correct twenty years ago, has undergone some change in the interval. 

 At that time Clark and Barlee found it in myriads between Green- 

 wich and Charlton ; but at the present time neither Mr. Jeffreys nor 

 myself can find it there. We have, however, found it in countless 

 thousands at Abbey Wood, and Erith, on the raised banks of the 

 Thames, which now seems its nearest locality to London, so that they 

 appear to have migrated a distance of about ten miles ;" and Mr. 

 Marshall adds that " as Mr. Horsley has been searching for this 

 species also at the latter stations without success, it must have 

 migrated further still, if the sewage outfall works of recent years has 

 not altogether exterminated it." 



A short time after this was written by Mr. ^Larshall, the Rev. J. 



