P I S 



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P L A 



Pisolite is nearly or quite opaque, ad 

 consists of small rounded masses, coin- 

 posed of concentric layers, each concre- 

 tion having a grain of sand for its nu- 

 cleus, or centre. These concretions, ag- 

 glutinated by a calcareous cement, form 

 masses of considerable magnitude, and 

 sometimes continuous beds. 



PISOLI'TIC. Composed of pisolite ; con 

 taining pisolite ; resembling pisolite. 



PI'STACITE. Another name for epidote. 

 See Epidote. 



PI'STAZITE. See Epidote. 



PI'STIL. (from pintillum, Lat. pistil, Fr.) 

 In botany, the female organ of the plant ; 

 situated in the centre of the flower, and 

 forming the rudiments of the fruit. A 

 perfect pistil is composed of three parts, 

 the ovarium, the style, and the stigma. 

 Each modified leaf which forms the pis- 

 til, is called a carpellum : the carpella 

 are so folded that the margins of the leaf 

 are next to the axis, or centre ; and from 

 these a species of bud is produced, which 

 is the seed. The form of the pistil must 

 depend on that of the carpella, on their 

 number, and on their arrangement. 



PITCH -STONE. A vitreous lava, of a black- 

 ish-green, or a nearly black colour ; a 

 semi-vitreous substance having the lustre 

 and appearance of pitch, and containing 

 a portion of bitumen. Specific gravity 

 from 2-29 to 2-64. Before the blow-pipe 

 it whitens, tumesces, and fuses into a 

 porous, whitish enamel. It consists of 

 silex 73*0, alumine 14'5, soda 1'75, lime 

 I'O, oxides of iron and manganese 1*1, 

 water 8 '.50. Pitch-stone occurs in veins 

 and in beds ; sometimes forming whole 

 mountains. 



PIT COAL. The coal usually consumed in 

 our houses, and thus called from its 

 being dug out of pits. See Coal. 



PLACE'NTA. (placenta, Lat. a cake, pla- 

 centa, Fr.) 



1. In anatomy, the medium of commu- 

 nication between the mother and the 

 foetus. 



2. In botany, that part of the ovarium 

 to which the seeds are attached. 



3. In fossilogy, the name given by Klein 

 to a section of catocysti, from the shells 

 being flat, like a cake. They are all or- 

 namented with a pentaphylloid flower. 

 The mouth is in the middle of the base, 

 and the anal orifice near the margin. 

 Placenta are divided by Klein into three 

 genera, mellita, laganum, and rotula. 



PLACOI'DIAN. (from 7r\d, a broad plate, 

 and tt^oc, form, Gr.) One of the orders 

 into which M. Agassiz divides the class 

 of fishes. The placoidians are distin- 

 guished by their skin being irregularly 

 covered with plates of enamel. In this 

 order are comprised all the cartilaginous 



fishes of Cuvier, the sturgeon only ex- 

 cepted. 



PLACOI'DIAN. Belonging to the order of 

 Placoidians. 



PLAGIO'STOMA. (from TrXayiof, oblique, 

 and ffrofjia, mouth, Gr.) A genus of 

 sub - equivalve, inequilateral, oblique 

 shells, known only in a fossil state. The 

 plagiostoma is one of the most charac- 

 teristic shells of the chalk formation. 

 Several species are known, namely, P. 

 spinosum, P. Hoperi, P. elongatum, P. 

 asper, &c. The Plagiostoma spinosum 

 for the long slender spines attached to 

 its upper valve. The spines arise from 

 costae, which radiate from the beaks to 

 the margin, but without any regularity : 

 they vary in number from fifteen to 

 twenty, and some of them are two inches 

 and upwards in length. They are very 

 numerous in the chalk pits between 

 Shoreham and Bramber, but there is 

 great difficulty in extricating them from 

 the surrounding chalk without breaking 

 the spines. 



PLANO'RBIS. (from planus, flat, and orbis, 

 an orb, Lat.) A genus of discoidal, 

 fresh-water univalves, resembling the am- 

 monite, but not chambered. Planorbis 

 belongs to the family Pulmonea in Cu- 

 vier's arrangement, and to Lymnacea in 

 Lamarck's and Blainville's. Planorbes 

 may be distinguished from helices by the 

 slight increase of the whorls of their 

 shell, by the convolutions 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. 



From the delicacy of their structure, 

 as well as the slight degree of mineraliza- 

 tion they have undergone, planorbes are 

 rarely obtained as fossils in a perfect 

 state. Three species have been found in 

 the neighbourhood of Paris. Two spe- 

 cies are mentioned as found in the allu- 

 vial deposites of Sussex ; and planorbes 

 are also found in the fresh-water strata 

 of the Isle of Wight. 



PLA'SMA. A grass-green variety of rhom- 

 bohedral 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 ornamental articles of dress. 



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

 gypsum. See Gypsum. 



PLA'STER STONE. Another name for plaster 

 of Paris. 



PLA'STIC CLAY, (from TrXaoriKoc, Gr. fit 

 for the art of fashioning, plastiquc, Fr.) 

 A name given to one of the beds of the 

 eocene period, from its being used in the 

 manufacture of pottery. The plastic clay 



