Disappearing Downland snails

Letter to the Sussex County Magazine, 1930: ‘Can any reader suggest a reason for the disappearance of the tiny white snails which, 25 or 30 years ago, were to be seen in thousands on certain hills of the Downs?

‘These, I believe, were known as the Little White snail, and were about the size of a pearl shirt button.

‘In my boyhood days I have seen some of the Downs literally specked with white, owing to the presence of tens of thousands of these 

‘The shell was milky white and was marked with a narrow black spiral 
line. Why have they disappeared.’

Why indeed!