Oriental sore, [121]

Ornamentation of shell-wall in Foraminifera, [66]

Oscarella, [196], [213], [225];

A. lobularis, [230]

Osculosa (Monotrypasta), [76]

Osculum, of Radiolaria, [76];

of Phaeodaria, [76], [82];

of Sponges, [169], [171], [174], [188], [189]

Ossicles of Holothuroidea, [569]

Ostium, [169]