Source : Webster's Revised Unabridged Dictionary (1913)

Orthoceras \Or*thoc"e*ras\, n. [NL., fr. Gr. 'orqo`s straight +
   ? a horn.] (Paleon.)
   An extinct genus of Paleozoic Cephalopoda, having a long,
   straight, conical shell. The interior is divided into
   numerous chambers by transverse septa.