L A 



L 3 56 J 



P L A 



into two sections, one of which has 

 been shewn by M. Deshayes to 

 pertain to the Lima, the other, 

 from which have been constructed 

 the three genera Podopsis, Pachy- 

 tes, and Dranchora, has likewise 

 been proved by M. Deshayes to be 

 live spondyli; the genus Plagios- 

 toma is therefore useless and should 

 expunged. The well known Pla- 

 giostoma Spinosune of the chalk is 

 a Spondylus. Lycett. 



PLANO'BBIS. (from planus, flat, and 

 orlis, an orb, Lat.) A genus of 

 discoidal, fresh-water univalves, 

 resembling the ammonite, but not 

 chambered. Planorbis belongs to 

 the family Pulmonea in Cuvier's 

 arrangement, and to Lymnacea in 

 Lamarck's and Blainville's. Pla- 

 norbes may be distinguished from 

 Helices by the slight increase of the 

 whorls of their shell, by the con- 

 volutions being nearly in one plane, 

 and by the aperture being wider 

 than it is high. All the shells of 

 this genus are reversed : they 

 abound in pools and ditches. 



PLAN'TIGEADE. A term applied to the 

 feet of animals when so constructed 

 as to allow the sole, from the heel 

 to the toes, to be fairly applied to 

 the surface of the ground. 



PL'ANULITE. The name assigned by 

 Lamarck to a genus of univalve 

 fossils formerly considered as a 

 species of ammonites. The planu- 

 lite differs from the ammonite in 

 not being articulated. 



PLA'SMA. (from TrXao/ia, Gr. image, 

 this stone having been formerly 

 used for engraving.) A grass-green 

 variety of rhombohedral quartz. 

 Fracture conchoidal ; lustre feeble 

 and resinous. It occurs in beds 

 associated with common chalcedony. 

 It is brought from Italy and the 

 Levant : it was worn by the 

 Romans, and formed into orna- 

 mental articles of dress. By some 

 mineralogists, plasma is considered 

 to be a variety of calcedony. Spe- 



cific gravity 2*55. Before the 

 blow-pipe it is infusible, but parts 

 with its colour, and becomes 

 whitish. According to Klaproth, 

 it consists of silica 96-75, alumina 

 0-25, iron 0'50. 



PLA'STER OF PA'RIS. A sub-species 

 of gypsum. See Gypsum,. 



PLA'STIC CLAY, (from 7r\affTiicb<?, Gr. 

 fit for the art of fashioning, 

 plastique, Fr.) The argille plas- 

 tique of the French geologists. A 

 name given to one of the beds of 

 the eocene period, from its easily 

 receiving and preserving the forms 

 given to it. The plastic clay 

 comprises the lowest formation of 

 deposits of the Eocene period. The 

 plastic clay of the Paris basin is 

 described as sometimes consisting 

 of two beds, separated by a bed of 

 sand. The lower bed is properly 

 the plastic clay. It is unctuous, 

 tenacious, contains some silicious 

 but no calcareous matter, and is 

 absolutely refractory in the fire, 

 when it has not too great a portion 

 of iron. It varies much in colour, 

 being very white, grey, yellow, 

 grey mixed with red, and almost 

 pure red. This clay is employed, 

 according to its quality, in making 

 a coarse and fine pottery and 

 porcelain. The French sands are 

 of great variety of colours. A 



ries of imperfect coal occurs in 

 lower strata of the Paris basin. 

 The sands of the formation in this 

 country are of almost every variety 

 of colour, and this is more particu- 

 larly seen at Alum Bay, in the 

 Isle of Wight, where they are 

 employed in forming ornamental 

 articles resembling landscapes, by a 

 tasteful arrangement of them in 

 glass cases. The plastic clay 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. In some parts of 

 England, more particularly the 



