Middle Ordovician – Late Silurian
Armenoceras is an extinct genus of nektobenthic carnivores
Identification in Hand Sample:
- Straight shell
- Siphuncle off center
- Endosiphuncular canals well developed, commonly complex
- Siphuncle is segmented and abuptly expanded
[leaflet-map lat=39 lng=-84.5 zoom=7 width=575 height=250 align="right"]
Treatise on Invertebrate Paleontology, Part K, Vol. 3 (1964):
- Medium-sized to large straight shell. Siphuncle off center, large; adnation areas broad; septal necks very short, brims wide and in contact with posterior surface of septa; endosiphuncular canals well developed, commonly complex. Cameral deposits rare.
- Armenoceras is differentiated from Actinoceras by the segments of the siphuncle, which are more abruptly expanded, so that the brim is well developed and the neck very short. The brim is sometimes free and sometimes recumbent, that is, so recurved as to lie in contact with the free part of the septum of which it is a modified continuation, and all gradations between the two conditions exist. The segments are expanded within the camerae, rounded, and generally the adapical end of the connecting ring is adnate to the septum. Annulosiphonate deposits are typical of the actinoceroids. The course of the radial canals varies somewhat within the genus, but consists typically of two series of arclike canals which are possibly efferent and afferent vessels, as in Nybyoceras. Teichert (1933) has discussed this type of structure more adequately. However, some species of this genus appear to have simpler canals, and Flower (1941) has suggested that perhaps these forms are more closely allied to the Sactoceratidae.
- Preceding the zone of expansion lie segments that are rather variable in form. Kobayashi’s (1937, pl. 1, fig. 2) figure of Selkirkoceras yokusense (Kobayashi) shows in the apical segments the recumbent brim and the strong area of adnation that characterize Armenoceras.