Endocerotoidea

a superorder of extinct cephalopod mollusks. The Endocerotoidea lived during the Ordovician. Their shell was external and straight, conical, or, more rarely, curved. Sometimes it was very large, as much as 9 m. The Endocerotoidea differ from other cephalopod mollusks by their very broad siphon, which in some forms was two-thirds the size of the shell’s diameter and included a substantial portion of the animal’s soft tissue. The Endocerotoidea inhabited the seas and were probably benthic. They are important in the stratigraphy of Ordovician deposits and in elucidating the evolutionary paths of cephalopod mollusks.