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. polus, many; pous, foot).  Restricted to the single individual of a simple Actinozooen, such as a Sea-anemone, or to the separate zooids of a compound Actinozooen.  Often applied indiscriminately to any of the Coelenterata, or even to the Polyzoa.

POLYPORA (Gr. polus, many; poros, a passage).  A genus of Lace-corals (Fenestellidoe).

POLYTHALAMOUS (Gr. polus; and thalamos, chamber).  Having many chambers; applied to the shells of Foraminifera and Cephalopoda.

POLYZOA (Gr. polus; and zooen, animal).  A division of the Molluscoida comprising compound animals, such as the Sea-mat—­sometimes called Bryozoa.

PORIFERA (Lat. porus, pore; and fero, I carry).  Sometimes used to designate the Foraminifera, or the Sponges.

PRAEMOLARS (Lat. proe, before; molares, the grinders).  The molar teeth of Mammals which succeed the molars of the milk-set of teeth.  In man, the bicuspid teeth.

PROBOSCIDEA (Lat. proboscis, the snout).  The order of Mammals comprising the Elephants.

PROCOELOUS (Gr. pro, before; koilos, hollow).  Applied to vertebrae the bodies of which are hollow or concave in front.

PRODUCTA (Lat. productus, drawn out or extended).  An extinct genus of Brachiopods, in which the shell is “eared,” or has its lateral angles drawn out.