P L A 



[357] 



P L E 



western, the plastic clay has suffered 

 considerable destruction. Two iso- 

 lated patches of it may be observed 

 near Weymouth. one on Came 

 Down, on the Ridgeway, and the 

 other at Black Down. Viewed on 

 an extended scale, the plastic clay 

 is composed of an indefinite number 

 of sand, clay, and pebble beds, 

 irregularly alternating, the distri- 

 bution of the organic remains, like 

 the alternation of the strata, being 

 exceedingly variable : sometimes 

 they occupy the clay ; at other 

 times the sand or pebbles; and 

 very frequently are altogether 

 wanting in both. 



The Druid sandstones, grey 

 weathers, sarsenstones, and pud- 

 dingstones, scattered in loose blocks 

 over many of the chalk downs 

 around the London Basin, are be- 

 lieved by Mr. Prestwich to be con- 

 solidated portions of the sands and 

 gravels of the plastic clay series. 



PLA'STEON. (plastron, Fr.) A name 

 given to the sternum of reptiles. 



PLATE. A provincial term for shale, 

 or slate clay. This plate or shale 

 is found in the coal and associated 

 beds, intercalated with them. 



PLA'TINA. | (platina, Spanish, from 



PLA'TINUM. ) plata, silver.) A metal; 

 one of the simple or elementary 

 bodies. Platinum was not known 

 in Europe till Mr. Wood brought 

 some of it from America in 1741. 

 "W hen pure, it is of a white colour, 

 like silver, but not so bright. It 

 has neither taste nor smell. It is 

 exceedingly malleable and ductile ; 

 it may be hammered into plates of 

 extreme thinness, and Dr. Wollas- 

 ton succeeded in drawing out a 

 wire of this metal to the fineness 

 of l-10,000dth of an inch. Plati- 

 num is one of the most infusible of 

 metals, not yielding before the 

 utmost heat of the furnace ; it is 

 soluble in chlorine and nitro-muri- 

 atic acid. It was first obtained 

 from Choco and Santa Fe, in South 



America ; it has since been discov- 

 ered in the Brazils, Spain, and in 

 the Ural mountains, in Siberia. In 

 the ore of platinum four new 

 metals have been discovered, name- 

 ly, iridium, palladium, osmium, 

 and rhodium. 



PLATYCEINI'TES. (from TrXartW, broad, 

 and icplvov, a lily, Gr.) A genus 

 of the family crino'idea, of the 

 division, according to Miller's 

 arrangement, crinoidea inarticulata. 

 That author thus describes the gen- 

 eric characters of Platycrinites : 

 "A crinoidal animal, with an 

 elliptic, or (in one species) penta- 

 gonal column, formed of numerous 

 joints, having a few side arms at 

 irregular distances. Pelvis, saucer- 

 shaped, formed of three unequal 

 pieces, from which five large plate- 

 like scapula proceed. Base pro- 

 vided with numerous fibres for 

 attachment. The want of costae, 

 supplied by the large plate-like 

 scapulae, gives the superior part of 

 these animals a pentagonal appear- 

 ance, and furnishes so conspicuous 

 a character, that they are readily 

 distinguished from all other 

 genera." 



PLATYGNA'THUS. A genus of ichthy- 

 olites of the old red sandstone. A 

 jaw, in the possession of Dr. Traill, 

 that of an Orkney species of Pla- 

 tygnathus, does not exceed in bulk 

 the jaw of a full grown cod fish. 

 Hugh Miller. 



PLEC'TEODTTS. The name given by M. 

 Agassiz to a genus of fossil fishes 

 discovered in the Upper Ludlow 

 rock. The name has been chosen 

 from the circumstance of the teeth 

 being bristled with sharp points, 

 like the spurs of a cock. The teeth, 

 says M. Agassiz, cannot be referred 

 to any species already known, and 

 constitute a genus, the fishes of 

 which were without doubt the 

 pirates of the seas of that period. 

 If there is but one species, it might 

 be named Plectrodus mirabilis ; if 



