Serrated natural rubber septa 

Closure has a turnover flange molded to grip outside of container neck, promoting a double seal in conjunction with annular serrations on a hollow plug. Annular serrations depress themselves against the inside wall of the bottle neck, making each serration a suction sealing point.
Note: For extra security when using large bore transfer needles, fasten septum to bottle or flask opening with a wrap-it tie. See ties and accessory tool listings on next page.