NAUTILOID. Resembling the sh.e.l.l of the _Nautilus_ in shape.

NERVURES (Lat. _nervus_, a sinew). The ribs which support the membranous wings of insects.

NEUROPTERA (Gr. _neuron_, a nerve; _pteron_, a wing). An order of Insects characterised by four membranous wings with numerous reticulated nervures (_e.g._, Dragon-flies).

NEUROPTERIS (Gr. _neuron_, a nerve; _pteris_, a fern). An extinct genus of Ferns.

NOTHOSAURUS (Gr. _nothos_, spurious; _saura_, lizard). A genus of _Plesiosaurian_ Reptiles.

NOTOCHORD (Gr. _notos_, back; _chorde_, string). A cellular rod which is developed in the embryo of Vertebrates immediately beneath the spinal cord, and which is usually replaced in the adult by the vertebral column. Often it is spoken of as the "chorda dorsalis."

NUDIBRANCHIATA (Lat. _nudus_, naked; and Gr. _bragchia_, gill).

An order of the _Gasteropoda_ in which the gills are naked.

NUMMULINA (Lat. _nummus_, a coin). A genus of _Foraminifera_, comprising the coin-shaped "Nummulites."

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 sh.e.l.l 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 sh.e.l.l). An order of small Crustaceans which are enclosed in bivalve sh.e.l.ls.

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 const.i.tuted 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.

PALaeOTHERIDae. (Gr. _palaios_, ancient; _ther_, beast). A group of Tertiary Ungulates.

PALaeOZOIC (Gr. _palaios_, ancient; and _zoe_, life). Applied to the oldest of the great geological epochs.

PARADOXIDES (Lat. _paradoxus_, marvellous). A genus of Trilobites.

PATAGIUM (Lat. the border of a dress). Applied to the expansion of the integument by which Bats, Flying Squirrels, and other animals support themselves in the air.

PECOPTERIS (Gr. _peko_, I comb; _pteris_, a fern). An extinct genus of Ferns.

PECTEN (Lat. a comb). The genus of Bivalve Molluscs comprising the Scallops.

PECTORAL (Lat. _pectus_, chest). Connected with, or placed upon, the chest.

PENTACRINUS (Gr. _penta_, five; _krinon_, lily). A genus of Crinoids in which the column is five-sided.

PENTAMERUS (Gr. _penta_, five; _meros_, part). An extinct genus of Brachiopods.

PENTREMITES (Gr. _penta_, five; _trema_, aperture). A genus of _Blastoidea_, so named in allusion to the apertures at the summit of the calyx.

PERENNIBRANCHIATA (Lat. _perennis_, perpetual; Gr. _bragchia_, gill). Applied to those Amphibia in which the gills are permanently retained throughout life.

PERISSODACTYLA (Gr. _perissos_, uneven; _daktulos_, finger).

Applied to those Hoofed Quadrupeds (_Ungulata_) in which the feet have an uneven number of toes.

PETALOID. Shaped like the petal of a flower.

PHACOPS (Gr. _phake_, a lentil; _ops_, the eye). A genus of Trilobites.

PHALANGES (Gr. _phalanx_, a row). The small bones composing the digits of the higher _Vertebrata_. Normally each digit has three phalanges.