The class of Vertebrates comprising
the Fishes.
PLACOID (Gr. _plax_, a plate; _eidos_, form). Applied to the
irregular bony plates, grains, or spines which are found in the
skin of various fishes (_Elasmobranchii_).
PLAGIOSTOMI (Gr. _plagios_, transverse; _stoma_, mouth). The
Sharks and Rays, in which the mouth is transverse, and is placed
on the under surface of the head.
PLATYCERAS (Gr. _platus_, broad; _keras_, horn). A genus of Univalve
Molluscs.
PLATYCRINUS (Gr. _platus_, broad; _krinom_, lily). A genus of
Crinoidea.
PLATYRHINA (Gr. _platus_, broad; _rhines_, nostrils). A group
of the _Quadrumana_.
PLATYSOMUS (Gr. _platus_, wide; _soma_, body). A genus of Ganoid
Fishes.
PLEISTOCENE (Gr. _pleistos_, most; _kainos_, new). Often used
as synonymous with "Post-Pliocene."
PLEUROTOMARIA (Gr. _pleura_, the side; _tome_, notch). A genus
of Univalve shells.
PLIOCENE (Gr. _pleion_, more; _kainos_, new). The later Tertiary
period.
PLIOPITHECUS (Gr. _pleion_, more; _pithekos_, ape). An extinct
genus of monkeys.
PLIOSAURUS (Gr. _pleion_, more; _saura_, lizard). A genus of
Plesiosaurian Reptiles.
POLYCYSTINA (Gr. _polus_, many; and _kustis_, a cyst). An order
of _Protozoa_ with foraminated siliceous shells.
POLYPARY. The hard chitinous covering secreted by many of the
_Hydrozoa_.
POLYPE (Gr.
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