| Passive diffusion of sediment into closed ammonite chambers is unlikely due to sediment plugging of the ammonite siphuncle and lack of fluid through flow. |
| Shell straight or curved, fusiform, aperture simple, siphuncle contracted at septa. |
| This is a small group of specialized ellesmerocerids with a concave outline of siphuncle segments. |
| In Ammonoids the siphuncle is always marginal, and usually external. |
| The higher the value, the nearer the siphuncle is toward the concave side of the conch. |
| Within the first chamber the first segment of the siphuncle appears. |
