Knights Vs Snails

What's That About?

Recently a group of people working at the British Library went into their manuscripts store to have a look at some medieval genealogical rolls. They were examining an English roll from the last part of the 13th century which contains quite a lot of marginalia (scribbles, comments and illuminations in the margins of a book).

As anyone who is familiar with 13th and 14th century illuminated manuscripts can attest, images of armed knights fighting snails are common. Why is that? What do these images mean?