on_, 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 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 genu