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.

(Teichert 1964) Its primary diagnostic feature is its ventral siphuncle which the septa across obliquely.

(Teichert 1964) Flower (1957) described the canal system within the siphuncle as being primarily of the single arc type, like Armenoceras, and with interconnecting tubes.

Nybyoceras ranges from the Middle Ordovician (Chazyan equivalent) in northeast China to the early Upper Ordovician Cobourg-Eden-Red Stage tn North America, (Flower 1976) and according to Flower (1968, 1976) left no descendant genera.

Teichert on the other hand, earlier (1964) indicated that Nybyoceras gave rise to the Ordovician Selkirkoceras and to the Silurian Megadisocosorus.