Spirula spirula
Date 1910
Source Die Cephalopoden http://www.archive.org/details/diecephalopoden182chun
Author Ewald Rübsamen
Source: https://commons.wikimedia.org/wiki/File:Spirula_spirula.jpg

The ram's horn squid (Spirula spirula) is a species of deep-water squid-like cephalopod mollusk. It is the only extant member of the genus Spirula, the family Spirulidae, and the order Spirulida. Because of the shape of its internal shell, it is commonly known as the ram's horn squid or the little post horn squid. Because the live animal has a light-emitting organ, it is also sometimes known as the tail-light squid.

Live specimens of this cephalopod are very rarely seen, because it is a deep-ocean dweller. The small internal shell of the species is however quite a familiar object to many beachcombers. The shell of Spirula is extremely light in weight, very buoyant, and surprisingly durable; it very commonly floats ashore onto tropical beaches (and sometimes even temperate beaches) all over the world. This seashell is known to shell collectors as the ram's horn shell or simply as Spirula.

Superorder: Decapodiformes
Order: Spirulida
Suborder: Spirulina
Family: Spirulidae
Genus: Spirula
Species: Spirula spirula (Linnaeus, 1758)
Synonyms: Nautilus spirula Linnaeus, 1758