OBOLELLA (Lat. dim. of obolus, a small coin).  An extinct genus of Brachiopods.

OCCIPITAL.  Connected with the occiput, or the back part of the head.

OCEANIC.  Applied to animals which inhabit the open ocean (= pelagic).

ODONTOPTERYX (Gr. oduos, tooth; pterux, wing).  An extinct genus of Birds.

ODONTORNITHES (Gr. oduos, tooth; ornis, bird).  The extinct order of Birds, comprising forms with distinct teeth in sockets.

OLIGOCENE (Gr. oligos, few; kainos, new).  A name used by many Continental geologists as synonymous with the Lower Miocene.

OPHIDIA (Gr. ophis, a serpent).  The order of Reptiles comprising the Snakes.

OPHIUROIDEA (Gr. ophis, snake; oura, tail; eidos, form).  An order of Echinodermata, comprising the Brittle-stars and Sand-stars.

ORNITHOSCELIDA (Gr. ornis, bird; skelos, leg).  Applied by Huxley to the Deinosaurian Reptiles, together with the genus Compsognathus, on account of the bird-like character of their hind-limbs.

ORTHIS (Gr. orthos, straight).  A genus of Brachiopods, named in allusion to the straight hinge-line.

ORTHOCERATIDAE (Gr. orthos, straight; keras, horn).  A family of the Nautilidoe, in which the shell is straight, or nearly so.

ORTHOPTERA (Gr. orthos, straight; pteron, wing).  An order of Insects.

OSTEOLEPIS (Gr. osteon, bone; lepis, scale).  An extinct genus of Ganoid Fishes.

OSTRACODA (Gr. ostrakon, a shell).  An order of small Crustaceans which are enclosed in bivalve shells.

OTODUS (Gr. ota, ears; odous, tooth).  An extinct genus of Sharks.

OUDENODON (Gr. ouden, none; odous, tooth).  A genus of Dicynodont Reptiles.

OVIBUS (Lat. ovis, sheep; bos, ox).  The genus comprising the Musk-ox.

PACHYDERMATA (Gr. pachus, thick; derma, skin).  An old Mammalian order constituted by Cuvier for the reception of the Rhinoceros, Hippopotamus, Elephant, &c.

PALAEASTER (Gr. palaios, ancient; aster, star).  An extinct genus of Star-fishes.

PALAEOCARIS (Gr. palaios, ancient; karis, shrimp).  An extinct genus of Decapod Crustaceans.

PALAEOLITHIC (Gr. palaios, ancient; lithos, stone).  Applied to the rude stone implements of the earliest known races of men, to the men who made these implements, or to the period at which they were made.

PALAEONTOLOGY (Gr. palaios, ancient; and logos, discourse).  The science of fossil remains or of extinct organised beings.

PALAEOPHIS (Gr. palaios, ancient; ophis, serpent).  An extinct genus of Snakes.

PALAEOSAURUS (Gr. palaios, ancient; saura, lizard).  A genus of Thecodont Reptiles.