See text Tonna is a genus of large sea snails, marine gastropod molluscs in the family Tonnidae, the tun or cask shells.

The head is broad and swollen anteriorly, supporting two long, slender, obtuse, distant tentacles, dilated towards the base, where the eyes are situated.

[2] The genus Tonna comprehends a small number of species, some of which attain so remarkable a growth that they are sometimes as large as a man's head.

The general appearance of the shell, of an inflated, thick-set form, calls up the image of a tun, whence is derived its generic name.

[2] The animals of the tuns are in general strongly colored, and painted with different tints which form bands and spottings upon their entire exterior.