Sea To Summit says:

We were the first company to introduce a compression dry bag into the market which does not use a purge valve. This innovation uses air permeable eVent® fabric on the base, which allows air to be pushed out of the sack, while stopping water getting in. This is a lighter, simpler and more reliable solution than a purge valve. The body is made of a tough 70D polyurethane-coated fabric.

It has a Hypalon roll-top closure like the rest of our dry sacks range and a lid with four straps for even compression.