''Angelinoceras'' is a genus of lituitids from the Middle Ordovician that starts off with an open spiral of about 1.5 strongly compressed whorls followed by a straight

orthoconic An orthocone is an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named ''Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloids d ...

section that continues to expand for a length about equal to the diameter of the coiled portion before retaining a more or less consistent diameter. The juvenile spiral portion has a deep indentation on the ventral side of the aperture for the water-jet funnel, known as a hyponomic sinus, that becomes broad and shallow in the straight-shelled adult portion. ''Angelinoceras'' is most likely derived from ''

Holmiceras ''Holmiceras'' is one of two ancestral lituitids from the late Early Ordovician (Arenigian). The other being ''Ancistroceras''. ''Holmiceras'' begins with about 1.5 to 2 loosely coiled whorls followed by a rapidly expanding orthocone, much like ...

'' by a further opening of the initial spiral and a reduction in the expansion of the

orthocone An orthocone is an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named ''Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloids d ...

''References''

*Flower, R. H. 1950. A Classification of the Nautiloidia. Jour Paleontology, V.24, N.5, pp 604–616, Sept. *Furnish & Glenister, 1964. Nautiloidea -Tarphycerida. Treatise on Invertebrate Paleontology Part K, Mollusca 3 ...Nautiloidea {{Taxonbar, from=Q4762768 Prehistoric nautiloid genera