P L A 



[ 205 ] 



P L U 



and the London clay are deemed by some 

 geologists as one formation, and, although 

 separated by others, the line of separation 

 appears to be quite arbitrary. The plas- 

 tic clay is in some places of great thick- 

 ness, amounting to four or five hundred 

 feet ; in other parts, where present, it is 

 very thin. It is a marine deposit, oysters 

 and other marine shells being found 

 in it. 



PLA'STRON. (plastron, Fr.) A name given 

 to the sternum of reptiles. 



PLATI'NA. } (platina, Spanish, from plata, 



PLATI'NUM. $ silver.) A metal, consti- 

 tuting one of the fifty-five simple or ele- 

 mentary bodies. Platinum was not known 

 in Europe till Mr. Wood brought some of 

 it from America in 1741. When 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 ex- 

 treme thinness, and Dr. Wollaston suc- 

 ceeded in drawing out a wire of this metal 

 to the fineness of 1- 10,000dth of an inch. 

 Platinum is one of the most infusible of 

 metals, not yielding before the utmost 

 heat of the furnace ; it is soluble in chlo- 

 rine and nitro-muriatic acid. It was first 

 obtained from Choco and Santa Fe, in 

 South America ; it has since been dis- 

 covered in the Brazils, Spain, and in the 

 Ural mountains, in Siberia. In the ore 

 of platinum four new metals have been 

 discovered, namely iridium, palladium, 

 osmium, and rhodium, 



PLEROSA'URUS. A fossil saurian of the lias 

 and oolite. 



PLESIOSAU'RUS. (from 7rXr/<7W, near to, 

 and vavpa, a lizard, Gr.) A genus of 

 extinct amphibious animals, nearly allied 

 to the Ichthyosaurus. Cuvier says 

 this inhabitant of the ancient world, is 

 perhaps the most heterogeneous, and ap- 

 pears to merit the name of monster above 

 all others. It united the teeth of a cro- 

 codile to the head of a lizard ; its neck 

 was of enormous length, exceeding that of 

 its body, and resembling the body of a 

 serpent ; it possessed a trunk and tail 

 of the proportions of an ordinary qua- 

 druped ; to all these were added the pad- 

 dles of a whale. The teeth were conical, 

 very slender, curved inwards, finely stri- 

 ated on the enamelled surface, and hollow 

 throughout the interior. Five or six 

 species of the plesiosauri are known ; 

 they appear to have lived in shallow seas 

 and estuaries, and, in the opinion of 

 some, they swam upon or near the sur- 

 face, having the neck arched, like the 

 swan, and darting it down at the prey 

 within reach. Prodigious numbers of 

 remains are found in the lias. Ver- 

 tebree and teeth are found in the Hastings 



beds. Some of the plesiosauri were up- 

 wards of twenty feet long. 



PLEUROCY'STI. The third class of echini. 



PLEUROTOMA'RIA. A fossil genus of tur- 

 binated, spiral, univalve shells belonging 

 to the family Turbinacea, They are 

 found only fossil, and occur in the in- 

 ferior oolite. 



PLI'CATED. (from plico, Lat. to fold.) 

 Plaited ; folded. 



PU'OCENE. (from TrXeiuv, more, and 

 icaivoQ, recent, Gr.) The name given by 

 Mr. Lyell to a division of the supracre- 

 taceous group, or tertiary strata. The 

 tertiary series Mr. Lyell divided into four 

 principal groups, namely, the eocene, the 

 miocene, the older pliocene, and the 

 newer pliocene, each characterized by 

 containing a very different proportion of 

 fossil recent species. The newer pliocene, 

 the latest of the four, contains from 

 ninety to ninety-five per cent, of recent 

 fossils ; the older pliocene contains from 

 thirty-five to fifty per cent, of recent 

 fossils ; the miocene contains eighteen 

 per cent, of recent fossils ; the eocene 

 contains only three and a half per cent, of 

 recent fossils. In the newer pliocene de- 

 posites of the valley of Elsa, in Tuscany, 

 six living species of testacea were recog- 

 nized by M. Deshayes. The newer plio- 

 cene period is that which immediately 

 preceded the recent era ; the older plio- 

 cene period is that which intervened be- 

 tween the miocene and the newer plio- 

 cene. The newer pliocene formations 

 occur in Sicily and Tuscany ; the older 

 pliocene at Nice, Perpignan, Norfolk, 

 Suffolk, and near Sienna. Both the 

 newer pliocene and the older pliocene 

 exhibit marine as well as fresh-water 

 deposites. 



PLUMBA'GO. (plumbago, Lat.) Graphite. 

 Commonly called black-lead. See Black 

 Lead. 



PLU'MOSE. ) (plumosus, Lat. full of fea- 



PLU'MOUS. ) thers.) Feathery ; downy. 



PLTJ'MULE. (from plumula, Lat. a little 

 feather.) In botany, that part of the 

 seed which grows into the stem and axis 

 of the future plant. In the bean, horse- 

 chesnut, &c. the plumule is distinctly 

 visible, but in plants generally, it is 

 scarcely perceptible without the aid of a 

 magnifying glass ; and in many it does 

 not appear till the seed begins to ger- 

 minate. The first indication of develop- 

 ment, whenever the seed begins to ger- 

 minate, is the appearance of the plu- 

 mule, which is a collection of feathery 

 fibres, bursting from the enveloping cap- 

 sule of the germ, and which proceeds im- 

 mediately to extend itself vertically up- 

 wards. 



PLUTO'NIC. (from Pluto, one of the hea- 



