ORBIT.—The bony cavity for the reception of the eye.
ORGANISM.—An organised being, whether plant or animal.
ORTHOSPERMOUS.—A term applied to those fruits of the Umbelliferæ which have the seed straight.
OSCULANT.—Forms or groups apparently intermediate between and connecting other groups are said to be osculant.
OVA.—Eggs.
OVARIUM or OVARY (in plants).—The lower part of the pistil or female organ of the flower, containing the ovules or incipient seeds; by growth after the other organs of the flower have fallen, it usually becomes converted into the fruit.
OVIGEROUS.—Egg-bearing.
OVULES (of plants).—The seeds in the earliest condition.
PACHYDERMS.—A group of Mammalia, so called from their thick skins, and including the elephant, rhinoceros, hippopotamus, &c.
PALÆOZOIC.—The oldest system of fossiliferous rocks.