PEA 



197 ] 



PEC 



in the fleshy part, near the hinge. For 

 one pearl that is found perfectly round 

 and detached between the membranes of 

 the mantle, hundreds of irregular ones 

 occur attached to the mother-of-pearl; 

 these are sometimes in such numbers as 

 to prevent the animal from closing its 

 valves, and thereby cause its destruction. 

 The pearl is supposed by some writers 

 to be the effect of disease ; it is a forma- 

 tion forced upon the oyster by some ex- 

 traneous substance within the shell, which 

 it covers with mother-of-pearl. Sir 

 Everard Home considered that the abor- 

 tive eggs of the animal were the nuclei 

 upon which the pearls were formed. 



To collect the pearl oysters, divers are 

 employed ; these men, provided with 

 baskets, descend to the bed at the bottom 

 of the sea, and during their stay there, 

 which does not exceed two minutes, ge- 

 nerally a minute and a-half, collect into 

 their baskets every thing they can grasp, 

 when they are rapidly, at a signal given, 

 hauled up to the surface. When the bed 

 is richly stored, a diver will collect 150 

 oysters at one dip, and a single diver will, 

 in one day, bring up from 1000 to 4000 

 oysters. Rev. W. Kirly. Bridgewater 

 Treatise. 



PEARL SI'NTER. Called also fiorite. A 

 variety of siliceous sinter of a white or 

 grey colour, found in volcanic tuff. 



PEARLA'CEOUS. Resembling mother-of- 

 pearl. 



PEA'RLSTONE. An igneous or volcanic 

 rock with a mother-of pearl lustre. The 

 Perlstein of Werner ; Obsidienne perlee 

 of Brongniart; Lave vitreuse perlee of 

 Haiiy. Pearlstone is a variety of obsidian 

 occurring in globular and concentric la- 

 mellar, iridescent, translucent concre- 

 tion. It scratches glass. Specific gra- 

 vity from 2-20 to 2-55. When breathed 

 upon, it frequently gives out an argilla- 

 ceous odour. Tts constituents are silica 

 77'0, alumina 13'0, lime and natron 2'6, 

 potash 1*4, oxides of manganese and iron 

 2-0, water 4-0. 



PEA STONE. A variety of limestone, called 

 also pisolite. It occurs in globular or 

 spheroidal concretions of the size of a pea. 

 See Pisolite. 



PEAT, (derived by some from the German 

 word pfutze, a pool, or standing water.) 

 An intermediate substance between simple 

 vegetable matter and lignite, the conver- 

 sion of peat into lignite being gradual, 

 and brought about by the action of water. 

 Peat is composed of the remains of many 

 different plants, but probably & great 

 portion is derived from the Sphagnum 

 palustre, and the process by which these 

 vegetables are thus converted is clearly 

 seen in the sphagnum palustre. As the 



lower extremity of the plant dies, the 

 upper sends forth fresh roots, thus fur- 

 nishing a perpetual supply of decomposing 

 vegetable matter. Dr. Maculloch states, 

 " where the living plant is still in contact 

 with the peat, the roots of the rushes, 

 and ligneous vegetables, are found vacil- 

 lating between life and death, in a spongy 

 half decomposed mass. Lower down, 

 the pulverized carbonaceous matter is 

 soon mixed with similar fibres, still re- 

 sisting decomposition. These gradually 

 disappear, and at length a finely-powdered 

 substance alone is found, the process 

 being completed by the total destruction 

 of all the organised bodies. The genera- 

 tion of peat, when not completely under 

 water, is confined to moist situations, 

 where the temperature is low, and where 

 vegetables may decompose without putri- 

 fying.' 1 



Sir H. Davy states that one hundred 

 parts of dry peat contain from sixty to 

 ninety-nine parts of matter destructible 

 by fire. One-tenth of the whole of the 

 surface of Ireland is stated to be peat. 

 At the bottom of peat-mosses there is 

 occasionally found a cake or pan of oxide 

 of iron ; whence this is derived does not 

 appear to be clearly understood. The 

 preservative property of peat is very re- 

 markable ; bodies of persons who have 

 perished in peat-bogs have been kept free 

 from putrefaction for many years. 

 PE'CTEN. (pecten, Lat. a comb.) A genus 

 of marine bivalves, belonging to the family 

 Ostracea, or, according to Lamarck's 

 arrangement, to the family Pectenides. 

 The pecten is a fossil as well as a recent 

 shell, many species being found in our 

 seas. Pecten s are found at depths vary- 

 ing to twenty fathoms, in mud, sandy 

 mud, and sand. It is a regular, eared, 

 longitudinally ribbed, inequivalved bi- 

 valve, with contiguous beaks, having a 

 triangular auricle on each side of the um- 

 bones. Hinge toothless ; pit trigonal. 

 One muscular impression. Fossil pectens 

 are found in the neighbourhood of Paris, 

 and in many parts of England ; in the 

 Harwich cliff; in the green-sand of Wilt- 

 shire ; near Thame in Oxfordshire ; in 

 Gloucestershire ; and in Sussex. Amongst 

 the fossils of Sussex one species of pecten 

 is found in the chalk ; four species in the 

 chalk marl ; one species in the gait ; and 

 three species in the Shanklin sand. 



1. In conchology, resembling a comb ; 

 cut into regular, straight, segments like a 

 comb. 



2. In botany, applied to a pinnatifid leaf, 

 whose segments are extremely narrow, 

 resembling the teeth of a comb. 



