70 SHRUBSOLE : EROSION OF CERTAIN FRESHWATER SHELLS. 



hence appear that the Dee water would possess a solvent power 

 over shelly matter not possessed by the water from the Trent 

 Canal. The erosive action, however, I before remarked was 

 delayed for two or three months, whereas it might be expected 

 that this chemical action would be set up at once. The retard- 

 ation is doubtless due to the epidermis doing its work, and it is 

 not until the water has pierced this horny armour that it can 

 pursue its work of erosion. 



This view of the case is somewhat strengthened by two 

 other instances in which erosion is seen in shells from certain 

 waters hereabouts. Near Wrexham there is a pool, in which 

 LimncBa stagnalis is similarly eroded. The amount of lime 

 present varies somewhat with the supply of water — the average 

 will not exceed four grains in the gallon. The other instance 

 is that of Unio margaritifer from the upper waters of the Dee 

 at Llanderfel, the umbonal regions of which when living are 

 deeply corroded, while dead shells may be found in every stage 

 of dissolution, eaten away by the solvent action of the water. 

 The water, on examination, proved to contain only a trace of 

 lime, a very unusual circumstance, but explained by the fact 

 that the catchment basin of the river at this point is over 

 volcanic rocks which contain no soluble salts of lime. Another 

 tributary of the Dee, at a point lower down, contains ten or 

 twelve grains of lime in the gallon, and it is the mingling of 

 these two supplies that gives to our Chester supply an average 

 of three grains in the quantity mentioned. 



The following is a summary of the facts so far as at present 

 investigated : — 



Lime present per gall. Result. 



Water from R. Dee near Chester ... 3-00 grs. ... Acted strongly on shells. 



,, Wrexham ... ... 4'oo ,, ... ditto. 



,, R. Dee near Llanderfel 0*53 ,, ... ditto. 



,, Trent Canal 8*33 ,, ... No action on shells. 



In putting forward this theory as to the cause of the erosion 

 seen in certain shells being due to the low per-centage of lime 

 present in the water, and the water in consequence exerting a 



J.C, v., July, 1886. 



