SHRUBSOLE : EROSION OF CERTAIN FRESHWATER SHELLS. 67 



from some cause or other, they, without exception, developed 

 on their outer surface a series of circular pits or holes, which 

 continued to enlarge till several coalesced, usually along the 

 lines of growth, and often extending across the shell, form- 

 ing a deep groove, which penetrated to the lowest shell layer, 

 and continued to widen and spread as time went on. 



I found further that the new shell growth, which was then 

 in vigorous formation, might resist for two or three months, the 

 erosive action, or whatever it was, but that after that time its 

 powers of resistance gave way, and it was no longer safe from 

 the influences at work. Still I found the action continued to 

 spread, deepening, and widening the trenches, notwithstanding 

 the obvious efforts of the animal to counteract the influence at 

 work by sundry repairs, until in some cases the shell, I fear was 

 pierced, causing the death of its inhabitant. Here was presented 

 an unexpected problem. What was the cause of this erosion ? 

 It certainly did not originate in any want of food necessary to 

 promote shell growth, for apart from the quantity of food eaten, 

 the condition of the new shell negatived any such view; besides 

 it was the old growth of shell which was first attacked. In my 

 perplexity I sought council of those versed in the life history of 

 the mollusca, but with no definite result. The phenomenon 

 had often been observed, but no explanation was forthcoming. 

 Jeffreys wrote : " Various theories have been put forward to 

 account for this erosion. In the case of freshwater shells many 

 naturalists have supposed that it is caused by gaseous action 

 (sulphuretted hydrogen) ; some have attributed it to the attacks 

 of Myriapodous insects, others to other mollusca eating away 

 the calcareous matter for the purpose of constructing their own 

 shells."* Altogether, Jeffreys had so little faith in these sugges- 

 tious that he consulted Mr. Justice Grove on the subject, who 

 wrote an elaborate letter t in reply, suggesting that " it might 

 be owing to the want of homogeneity in the substance of the 



* ' British Conchology,' vol. i. Introduction, p. li. 



+ See letter in extenso, 'British Conchology,' vol. i. Intro., pp. lii, liii, liv. 



