ammonitoidea

from The Century Dictionary.

• A superfamily of tetrabranchiate cephalopods, including those which have an external shell of two principal layers, with an initial smooth chamber and the siphonal cavity extending forward. It includes most of the order Ammonea.

from the GNU version of the Collaborative International Dictionary of English.

• noun plural (Zoöl.) An extensive group of fossil cephalopods often very abundant in Mesozoic rocks. See ammonite.