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.
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