O. quadricornis, [725];

O. trigonus, [362]

Ostraciontidae, [724], [721]

Ostracodermi, [522] f., [149], [721]

Otocinclus, [595]

Otolithus, [663];

air-bladder, [304];

O. regalis, [361]

Oviducal gland (shell-gland), [400], [407]

Ovipositor, [408]