2/6 



EROSION OF THE SHELL 



CHAP, IX 



Erosion. — The fresh-water Mollusca 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 sets in near the apex (Fig. 192), where the life of the 

 shell may be regarded as weakest, and in bivalves 

 near the umbones. It is commonest in old shells, 

 and rarely occurs in the very young. So long as 

 the periostracum is present to protect the shell, 

 erosion cannot set in, but when once it has been 

 removed the shell is liable to the chemical changes 

 set up in its substance by water. There is abun- 

 dant evidence to show that erosion is caused by 

 pollution of water. Oat of many instances one 

 must suffice. In a certain stream near Boston, U.S., 

 numbers of Mollusca occurred, the shells of which 

 were very perfect and free from disease. Some 

 little way down stream an alkaline manufactory 

 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. Shrubsole, in 

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



Fig. 192.— Ex- 

 ample of an 

 eroded fresh- 

 water shell 



(Melaniacon- 

 fusa Dohrn, 

 Ceylon) . 



Water from 



"-7-,r' 



River Dee, near Chester 



. 3-00 grs. acted strongly on shells 



Wrexham . . 



4-00 grs. „ 



River Dee, near Llanderyel 



. 0-53 grs. M 



Trent Canal . 



8-33 grs. no action „ ^1 



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



2 Journ. of Conch, v. p. QQ. 



