''Holmiceras'' is one of two ancestral lituitids from the late Early

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

(Arenigian). The other being ''

Ancistroceras ''Ancistroceras'' is one of the two ancestral lituitids from the late Early Ordovician (Arenigian). The other being '' Holmiceras''. The shell is weakly annulate, starts off with 1.5 to 2 contiguous or slightly separated whorls followed by a rap ...

''. ''Holmiceras'' begins with about 1.5 to 2 loosely coiled whorls followed by a rapidly expanding orthocone, much like ''Ancistroceras'' except for the apical whorls and that Holmiceras is known only from the late Early Ordovician whereas ''Ancestroceras'' extends through the middle of the period. ''Holmiceras'' may have given rise to ''

Angelinoceras ''Angelinoceras'' is a genus of Lituitidae, lituitids from the Middle Ordovician that starts off with an open spiral of about 1.5 strongly compressed whorls followed by a straight orthoconic section that continues to expand for a length about equ ...

'' which begins with an expanding open spiral.

''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 The ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

Part K, Mollusca 3 ...Nautiloidea {{Taxonbar, from=Q16977357 Prehistoric cephalopod genera

See also

''References''