A bell jar was a container used to hold scientific samples or used for displaying things.
The shopkeeper Garos had some stuffed bird-like creatures in a bell jar in his curio shop when he was visited by Jonathan Archer and Trip Tucker in 2151. (ENT: "Civilization")
In 2364, a microbrain from Velara III was kept in a bell jar. William T. Riker referred to it as such when giving the order to the transporter chief to beam it back down to the planet's surface. (TNG: "Home Soil")