‖n. [ NL., fr. Gr. 'orqo`s straight + ke`ras 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. [ 1913 Webster ]

n. [ Ortho- + Gr. &unr_;, &unr_;, a horn. ] (Zool.) An orthoceras; also, any fossil shell allied to Orthoceras. [ 1913 Webster ]