76 



EROSION OF THE SHELL 



Urosio7i. — The fresh -water MoUusca generally, and marine 

 mollusca in a few rare cases {Purpura, Littorina^ are subject to 

 erosion, or decay in the shell substance. In univalves erosion 

 usually seta in near the apex (Fig. 192), where the life of the 

 shell may lie regarded as weakest, and in bivalves 

 near the umbones. It is commonest in old shells, 

 and rarely occurs in tlie very young. So long as 

 the periostracuni is present to protect the shell, 

 erosion cannot set in, but when once it has been 

 removed the shell is lialjle to the chemical changes 

 set up in its suljstance Ijy water. There is 

 abundant evidence to show that erosion is caused 

 by pollution of water. Out of many instances 

 one must suffice. In a certain stream near Boston, 

 Ft 19-'> — e - U.S., numbers of Mollusca occurred, the shells of 

 ample of an wliicli were Very perfect and free from disease, 

 water*^ shell *^o^^*^ little Way down stream an alkaline manu- 

 factory drained its refuse into the water. At and 

 below this point for some distance every shell was 

 more or less eroded, most of them seriously. 

 Farther down, when the alkali refuse became diluted away, the 

 shells retained their normal condition.^ 



A small percentage of lime in the water appears to produce 

 erosion. The result of some experiments by G. W. Shruljsole, in 

 the investigation of this point, may be tabulated as follows : - — 



water 

 (Mdaniacon- 

 fusa Doliru, 

 Ceylon). 



^ J. Lewis, Proc. Bost. Soc. vi. p. 149. 



Journ. of Conch, v. p. &Q.. 



