Actincocerids (Order Actinocerida) and endocerids (Order Endocerida) are generally straight shelled nautiloid cephalopods that lived during the early and middle Paleozoic; Middle Ordovician to Early Pennsylvanian for the actinocerids, Early Ordovician to Silurian for the endocerids. Externally they were mostly similar although one branch within the Endocerida has downwardly curved, endogastric shells, reverse of the progenetors of coiled formed, and some within the Actinocerida became broad and rather flattened. Both, especially for the straight shelled forms, lived with a horizontal orientation as detarmed by the common generlly ventral siphuncles found in both and the apical counterbalancing internal deposits. This is where the similarities ended.
Internally actinocerids and endocerids are rather different. This is especially true of their siphuncles. Actinoceratids were heavy, with cameral deposits and expanded siphuncle segments containing deposits pierced by a complex canal system. Endoceratids on the other hand were light, the only significant mass being in the endocones found in the more after part of an often wide siphuncle. In the endocerids the viscera (body organs) probably occupied the forward, empty part of the siphuncle as well as the body chamber at the front of the shell while in the actinocerids the organs were restricted to the body chamber with the siphuncle containing only nerves, blood vessels and related tissue. With endocerids in general the siphuncle with endocones is often the only fossil that remains. On the other hand it is common to fine entire actinocerid phragmocones or segments there of. Obviously these were very different animals with different life styles .
