HARDWICKE'S SCIENCE-GOSSIP. 



naturalist, the plank which serves him for a bridge 

 refuses to support him when half across, or he fails to 

 leap properly and gracefully an extra-wide ditch, 

 which ends in his immersion in clear fluid or mud. 



The marshes between East Greenwich and Plum- 

 stead were frequently investigated between the years 

 1883-9, rendering it unnecessary to go over the same 

 ground again. 



Probably many years have elapsed since the 

 Thames Estuary was thoroughly worked by concho- 

 logists, and this is confirmed by the recent publication 

 of localities in which species have long ceased to exist, 

 and by the discovery of the new species of Hydrobia. 



A few details respecting the limits of various 

 species may be of interest to the readers of Science- 

 Gossip. The marsh brackish-water sliells consist of 

 six species, if we include Mr. Smith's new Hydro- 

 bia, which is now generally considered by eminent 

 conchologists, both at home and abroad, to be 

 worthy of specific rank. They are as follows : 

 Hydrobia uIvce, Penn, H. similis, Drap., II. ventrosa, 

 Mont., H. yeiikinsi, Smith, Assimiiiia Gi-ayana, 

 Leach, and Mchinpis tnyosotis, Mont. A pecuhar 

 dwarfed variety of Littoniia rudis also occurs with 

 H. vejitrosa in brackish water at Tilbury. 



Of these species, A. Grayana, M. myosotis and 

 H. ulva: are most marine in habit ; H. vaitrosa 

 inhabits ditches which are decidedly more brackish 

 than those which ^. similis and/I. Jenkinsi frequent. 



H. ulvcc may be taken alive upon mud, and in 

 partially dry ditches at Grays, Tilbury, and Gravesend, 

 by the riverside, and sparingly in brackish-water 

 ditches near Greenhithe village, in company with 

 H. vcnti-osa and A. Grayana. It has not yet been 

 taken higher up the river. 



Many years ago H. j/w/Z/j' inhabited ditches between 

 Greenwich and Woolwich, which were occasionally 

 flooded by the tide, and this locality has been given 

 by Mr. J. W. Williams in a recent work published in 

 1S89. This locality was no doubt correct in Jeffreys' 

 time, but they have (in company with other species) 

 long since been forced to migrate lower and lower 

 down the river, owing to the pollution of the ditches 

 by various factories, chemical and gas-works, and 

 Thames sewage. As far back as 1883 not even a 

 dead shell could be obtained from this locality. 



Industrious searching for this pretty little mollusc 

 has led us to the conclusion that this species is 

 doomed to speedy extinction in this district. It 

 seems always to have been peculiar in Britain to the 

 Thames marshes, and, like H. Jenkinsi, in all pro- 

 bability was originally introduced from abroad. It 

 appears to be limited to a single narrow ditch a few 

 hundred yards in length, and with two exceptions we 

 have not succeeded in finding them elsewhere. Once 

 a dead shell was taken with H. jfenkinsi from a ditch 

 at Beckton, and once a single live specimen with the 

 same species between Erith and Darenth Creek. In 

 the same ditch with H. similis may be found a 



number of H. ventrosa, a few Limncea tru7tcatula, 

 and dead shells only, of A. Grayana. 



Occasionally a few shells of H. similis have been 

 collected, which are of a clear, pellucid texture. Mr. 

 Marshall has proposed to call this variety V. Candida, 

 see "Journal of Conchology," vol vi. p. 141. It 

 has been deemed necessary to strictly preserve the 

 habitat of this rare species, so as not to be instru- 

 mental in its extermination as a British species. 



H. ventrosa inhabits in great abundance brackish- 

 water ditches between Erith and Gravesend, and 

 may be collected on the north bank of the Thames 

 at Purfleet, Grays, and Tilbury. The shells from 

 the different localities vary somewhat, but hardly 

 sufficient to be considered as distinct varieties. A 

 short and rather tumid form occurs in a ditch near 

 the river and training-ships at Grays. 



H. Jenkinsi is now, and for some years is likely to 

 be, the most abundant Hydrobia of the Thames 

 marshes. When collected in 1883 in ditches at East 

 Greenwich, it was fairly plentiful there ; two years 

 later, a few shells were taken at Plumstead, but they 

 were by no means common at that time. They are 

 now extinct between Greenwich and Woolwich, 

 owing to the same cause which forced H. similis 

 and A. Grayana to retreat lower down the river. At 

 certain periods the new species fairly swarm in the 

 ditches at Plumstead marshes, upon duck-weed, 

 chara, and the bright green ribbon-like weed Entero- 

 morpha intestinalis, Linn., which is so common in 

 brackish water. As mentioned in the above article, 

 they are a very active and hardy species, capable of 

 existing for prolonged periods in quite fresh, and 

 even in hard tap-water. 



They have been taken in winter from beneath the 

 ice, and hibernating in the banks of their habitat. 

 Like the other species, the shells from different 

 localities are extremely variable, and several forms 

 differ sufficiently from the type to be considered as 

 distinct varieties. One form in particular which 

 occurs with the type at Beckton is peculiar in having 

 a much shorter spire, and very tumid body whorl. 

 They are strongly carinated and tufted, and the 

 suture is somewhat deeper than the type. Upon th 

 dorsal side there is a considerable bulging out of the 

 penultimate whorl upon the left side, giving the 

 shell a distorted appearance. In this condition they 

 somewhat resemble enlarged H. similis. It has 

 been suggested that these examples are shells that 

 have been stopped in growth by the drying up of the 

 ditch, or some other cause. Provisionally it is 

 proposed to call this variety or monstrosity H. 

 Jenkinsi, V. tumida, Jenkins. This species now 

 exists in considerable abundance in ditches at Beckton, 

 and extends from the Arsenal wall at Plumstead to 

 a point midway between Darenth Creek and 

 Greenhithe. In all probability a few years will find 

 them extending down the river as far as Gravesend. 

 H. Jenkinsi was at one time mistaken for IT. similis, 



