Spyroceratidae
Encyclopedia
Spyroceratidae are defined (Sweet, 1964, K246) 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.
Pseudorthocerida
Pseudorthocerida are generally straight longiconic nautiloids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining.. Cameral deposits are common and concentrated ventrally...
with uniformly slender siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...
segments, longer than wide, that contract sharply near either end, in which endosiphuncular deposits first form a compete annulus before fusing ventrally.