ammonitoidea

from The Collaborative International Dictionary of English v.0.48

Ammonitoidea \Am*mon`i*toid"e*a\, n. pl. [NL., fr. Ammonite +
   -oid.] (Zool.)
   An extensive group of fossil cephalopods often very abundant
   in Mesozoic rocks. See {Ammonite}.
   [1913 Webster]