Spyroceratidae

Spyroceratidae Temporal range: M Silurian - Permian
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Nautiloidea
Superorder:	Orthoceratoidea
Order:	Pseudorthocerida
Family:	Spyroceratidae Shimazu & Obata 1935

Spyroceratidae are defined as Pseudorthocerida with uniformly slender siphuncle segments, longer than wide, that contract sharply near either end, in which endosiphuncular deposits first form a compete annulus before fusing ventrally.

...
Wikipedia