PLACENTA. at or near its marpin, is the umbilical cord. The lOrd is a whitish sumi-transparent structure of ail average length of about twenty inches and a diameter about that of the little finger. It con- sists of two arteries and a vein twisted upon one another and surrounded liy a peculiar substance called from its consistence the jelly of Wharton, which gives the cord its characteristic appear- ance. The function of the placenta is both respira- tory and alimentary. It aerates the fcetal blood by gaseous interchange of oxygen for carbon dioxide and other gaseous waste products. (See ClKCi I.ATIOX : and Blood.) In its nutritional function it supplies material for the rapid p-owtli of the fcetus, at the same time remov- ing the proilucts of tissue decomposition. The placenta is suliject to several pathological ab- normalities in form and attachment, to iiillamiiia- tioii. and to degenerative changes. For their detailed descriiition, and for the treatment of that dangerous form of misplacement known as placenta previa, consult: Jewett. The Prac- tice of Obntetrics (Xew York, liUX)); American Si/stems of Gyitcecijloqi/ and Obstetrics (ib., 1800). PLACENTA. In plants, the region to which ovules are attached in a plant ovary. It sometimes appears as a mere thickening of the walls, or as a more decided projection from the walls — |)arietal: sometimes several are united in a central column — axial. The>e distinctions are im])ortant distinguishing characters of certain natural orders. Tiie number of placenta" corre- sponds with double the number of carpels, each carpel noniially producing two rows of ovules. PLACENTIA. The ancient name for Pia- cenza (q.v. ). PLACE OF WAILING. An inclosed place near the Mosque of Omar in Jerusalem, where the .lews congregate on Friday to lament the fall of Israel and pray for its restoration. The wall at which they gather is believed to be the only remaining part of Solomon's temple. PLACETAS, pla-sfi'tas. A town of the Prov- ince of Santa Clara, Cuba, 22 miles southeast of the city of .Santa Clara, and connected by rail with the coast towns of Caibarien and Sagua la Grande (Map: Cuba, F 4). It is in the midst of an extensive sugar-producing I'egion. Its popu- lation in 1S99 was ,5400. PLACIDE, pla-scd', He.xry (1799-1870). A noted .iiicrican comedian, born in Charleston, S. C., September 8, 1799. lie was the son of a French dancer, and his training for the stage be- gan in childhood. After several yciirs in the Soutli he made his appearance in 182.3 at the Park The- atre, New York, as Zekiel Homespun, one of his most popular characters, and for the next twent.v .years he was a member of the Park Theatre company. Here and afterwards as a star he seems to have won the nnmingled admiration of the critics in roles as varied as those of the Fat Boy in Picl.iciel,-. Sir Harcourt Courtly in London Asmirancr. Sir Peter Teazle, Dr. OUapod, and (Jrandfather Whitehead. Aft«r his retire- ment in 180.5 he settled in Babylon. L. L, where he died, January 23, 1S70. Consult: Ireland, ftreords of the A'ei« York IStat/e (New York lS(i(i-07) ; Keese, A Group of Comedians (ib.', laoi). 82 PLAGIAULAX. PLACID'IA. See Honobius, and Valenti- KiAxrs III. PLACODEBM (from Gk. wXiJ, plax, tablet, plate -|- S^pfJ-a, derma, skin). An order under the subclass (ianoidei in the older classifications of fossil lisli. It included genera ill which the head and forward part of the trunk were covered bv heavy plates; for example, Pterichthys, Botb- riolepis, Coccosteus, Dinichthys. The group was based on superficial resemblances, and it has been broken up and the genera distributed among other orders. See Ostracodermi; Lungfi.sh ; Birxoi ; and articles on the genera mentioned above. PLACOIDEI, i>la-koi'd*-l (Neo-Lat. nom. pi., from <ik. TrXaKifiS?)!, iilakOdCs, Hat, from ir.^, plax, tablet, plate -|- eldos, cidos, form), or Placoio Fishes. An order of cartilaginous fishes, in the classification proposed by L. Agassiz, charac- terized bv having "placoid' scales, irregular plates of luird bone, not imbricated, but placed near to- gether in the skin. These scales or plates are of considerable size in some fishes, but in others they are verj' small tubercles, as in the dogfish, of which the skin forms fine-grained shagreen. The term is no longer in scientific use, PLACOPHORA (Xeo-Lat. nom. pi., from Gk, TrXdf, plax, plate, tablet + ^ipuv, phercin, to bear). A division of Mollusca, containing only the chitons. The modern chitons are descendants of a race which, though never represented by any considerable number of species or individuals, be- gan in the early Paleozoic and persisted with little cliange of form down to the present day. The first are known in the Ordovician rocks, they are scattered through the Silurian and Devonian, and they attained a slight prominence in the Car- boniferous. The Paleozoic species nuiiAer about 20, and the.y are distinguished from those of the Mcsozoic and Tertiary by the less perfect articu- lation of their valves and lesser degree of orna- mentation of the surfaces of their shells. Few iMesozoic species are known and the group is represented in the Tertiary by about 50 species that show vei-y close relationship to the modern forms. PLA'GIARY, Sir Fretful. A vain and irrit- able character in Sheridan's play The Critic, intended to ridicule Richard Cumberland, the dramatist. PLAGIAULAX. A genus of Jurassic mam- mals, tyiiiea! of the family (Plagiaulacidie), known only from lower jaws fossilized in the Knglish Purbeck beds, which imply an animal of the size of a small rat and representing the primitive multitubercu- late t.vpe of dentition. (See illustration.) It is PLAGIAULAX MINOR. Right lower jaw. showing lielieved the rodent-like "i"'*™ (m) and premolars incisor teeth did not grow from persistent pulps, and they have no anterior coating of enamel. The centre of the two molars on each side is hollowed, and the raised rim is beset with tubercles. Associated genera are Bolodon, Allodon, and Ctenaeodon, the last named based on lower jaws found in the Laramie formation of Wyoming,
