Helical forms, dextral or sinistral, loosely or tightly coiled; typically regular, but later whorls may be unstable; apical angle normally acute; in early species siphon in middle of exposed side, later migrating to upper margin or even upper internal angle of whorl; with strong ribs or tubercles or both; rarely smooth. Suture asymmetrical to accord with helical coiling; lobes primitively bifid but variable, tending to trifidity.

Reference:

Wright C.W. 1996. Treatise on Invertebrate Paleontology; Part L MOLLUSCS 4 Revised (Volume 4: Cretaceous Ammonoidea)