_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 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, 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.[352] [Illustration: FIG. 192.--Example of an eroded fresh-water shell (_Melania confusa_ Dohrn, Ceylon).] 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:[353]-- Water from Lime present Result per gall. River Dee, near Chester 3·00 grs. acted strongly on shells Wrexham 4·00 grs. „ „ „ River Dee, near Llanderyel 0·53 grs. „ „ „ Trent Canal 8·33 grs. no action „ CHAPTER X GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER MOLLUSCA--THE PALAEARCTIC, ORIENTAL, AND AUSTRALASIAN REGIONS