spirula

1 capitalized :  a genus (coextensive with a family Spirulidae of the order Decapoda) of small cephalopods related to the extinct belemnites, having a many-chambered shell coiled freely in a flat spiral that is comparable to the phragmocone of the belemnite shell and almost enveloped in the soft parts, and occurring in most tropic seas usually at great depths from which the shells float to the surface and are cast on beaches, although complete specimens of the animal are rare

2 plural -s :  a cephalopod of the genus Spirula