The Actinoceriatidae is a family of actinocerids

Actinocerida

The Actinocerida comprise an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial...

 named by Saemann in 1853 for those that grew to have large shells with blunt apecies and large 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...

s with widely expanded segments and a generally arcuate endosiphucular canal system. Their range is from the upper Middle Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 to the Lower Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

. Actinocerids are generally straight shelled nautiloid cephalopods with a siphuncle composed of expanded segments, typically with thin connecting rings, in which there are internal deposits penetrated by a system of canals

The Actinoceratidae are derived from Wutinoceras

Wutinoceras

Wutinoceras, orthoconic actinocerids with ventral siphuncles composed of broadly expanded segments.and the type genus of the Wutinoceratidae. -Distinguishing Characters:...

, possibly through an early Armenoceras

Armenoceras

Armenoceras is the principal genus of the Armenoceratidae, ranging from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian....

or through Nybyoceras

Nybyoceras

Nybyoceras is an actinocerid genus assigned to the Armenoceratidae and similar to Armenoceras except for having a siphuncle close to the ventral side of the shell.-Morphology:...

and give rise to Lambeoceras

Lambeoceras

Lambeoceras is a rather large actinocerid with a convexly lenticular cross section from the Upper Ordovician of North America, and the sole representative of the family Lambeoceratidae.-Morphological Description:...

and to the Huroniidae.

Seven genera are included in the Actinoceratidae, Actinoceras

Actinoceras

Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.- Morphology:...

, Floweroceras, Kochoceras, Leurorthoceras, Paractinoceras, Saffordoceras, and Troostoceras. Actinoceras is the earliest but with a range that carries it into the Lower Silurian. Troostoceras followed by Saffordoceras are later Middle Ordovician genera related to early Actinoceras. Kochoceras followed by Floweroceras are Upper Ordovian genera related to later Actinoceras. Leurothoceras and Paractinoceras, both from the Upper Ordovician, are shown to have their source in Middle Ordovician actinoceratids.

Paractinoceras is probably the most distinct actinoceratid from Actinoceras. Paractinoceras has a long, straight slender shell and a siphuncle that starts off like that in Actinoceras but becomes narrow like that in Ormoceras

Ormoceras

Ormoceras is an actinocerid genus of the family Ormoceratidae which is found in North America from the late Chazyan though the early Cincinnatian of the Middle and Upper Ordovician, but continued through the Devonian worldwide....

in the anterior part of the phragmocone. Kochoceras has a large blunt shell that expands more rapidly than Actinoceras and is stronly flattened on the ventral side causing possible confusion with Lambeoceras to casual observation. Floweroceras is doubtfully distinct from Kochoceras.

Leurothoceras is synonymous with Actinoceras

Actinoceras

Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.- Morphology:...

; Saffordoceras and Troostoceras are closely related.