Tonna is a genus of large sea snails, marine gastropod molluscs in the family Tonnidae, the tun or cask shells. Description The thin shell is ventricose, inflated, generally globular, rarely oblong and encircled with ribs. The spire is short. The outer lip is crenulated and sometimes denticulated throughout its whole length. The oblong aperture is very large and emarginated inferiorly. The columella is channeled. There is no operculum. The animal is very large, so as scarcely to be contained within its shell. The head is broad, swollen before, supporting two long, slender, obtuse, distant tentacles, dilated towards the base, where the eyes are situated. The mouth is large, muscular, strong and retractile. The respiratory tube is pretty stout. Its cavity is capacious, entirely open, provided with two branchiae placed on the left side, the larger of which, describes a pretty large semicircle. The trunk is cylindrical, very much developed, flexible, capable of being turned in every direction at the will of the animal, and of elongating itself in a remarkable manner. It is furnished internally with several rows of hooks. The foot is ovate, large, fleshy, bordering all parts of the shell. It is rounded, widened, lobed and dilated before, with a horizontal furrow. The posterior extremity has no trace of an. operculum. The generative organ of the male is very retractile. <ref name="Kiener">Kiener (1840). General species and iconography of recent shells : comprising the Massena Museum, the collection of Lamarck, the collection of the Museum of Natural History, and the recent discoveries of travellers; Boston :W.D. Ticknor,1837 (described as 'Dolium)</ref> 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. In fact 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. Thus, the characters which make up these species are a form more or less inflated, girdled, and very globular. The spire is much shorter than the body whorl. This causes the size of the aperture, which almost always occupies two thirds of the length of the shell. The animals of the tuns are in general strongly colored, and painted with different tints which form bands and spottings upon their entire exterior. Distribution This marine species is cosmopolitan. Species Species within the genus Tonna include: Tonna alanbeui