Shell vials are flat-bottomed vials used for making color comparisons among liquid substances. They are not intended to have a cap. If you wish to cap this vial, use a rubber stopper, cork or Parafilm. They come in three sizes. See more product details. Options: 8 mL, 7 cm, 2.1 cm. 16 mL, 6 cm, 1.7 cm. 30 mL, 9.5 cm, 2.5 cm.