PIS 



[ 355 J 



considerable magnitude, and some- 

 times continuous beds. 



PISOLI'TIC. Composed of pisolite ; 

 containing pisolite ; resembling pi- 

 solite. 



PI'STACITE. | Another name for epi- 



PI'STAZITE. } dote. 



PI'STIL. (from pistillum, 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 pistil, is called a car- 

 pellum : 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 pro- 

 duced, 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 

 blackish-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, alumine 

 14-5, soda T75, lime 1, oxides of 

 iron and manganese !!, water 

 8'50. Pitch-stone occurs in veins 

 and in beds, sometimes forming 

 whole mountains. 



PIT COAL. The coal usually con- 

 sumed in our houses, and thus 

 called from its being dug out of 

 pits. 



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

 placenta, Fr.) 



1. In anatomy, the medium of 

 communication 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 ornamented 

 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. 



PL'ACENTALIA. (from placenta, a cake, 

 Lat.) The name given to one of 

 the two great sections into which 

 the class mammalia is divided. 

 The orders comprised in this 

 section have been thus named from 

 a predominating feature in the re- 

 productive system of the animals 

 comprehended in them. Feetus 

 ad uterum maternum annexus per 

 medium placentas vera3, diverse* 

 conformatae, diversis mammalibus j 

 vel, inloco illius, membrana villosa, 

 et vasculari, x^/j/oi/ dicta. Martin. 



PLACOI'DEA. (from 7r\ag, a broad 

 plate, and eloo-s, form, Gr.) One 

 of the orders into which M. Agas- 

 siz divides the class of fishes. The 

 placoidians are distinguished by 

 their skin being irregularly co- 

 vered with plates of enamel. In 

 this order are comprised all the 

 cartilaginous fishes of Cuvier, the 

 sturgeon only excepted. 



PLACOTHO'EAX. A genus of ichthyo- 

 lites of the old red sandstone. 



PLAGIA'ULAX. (an abbreviation for Pla- 

 giaulacodon, from TrXafytos, oblique, 

 and av\a%, a groove, having re- 

 ference to the diagonal grooving of 

 the premolars.) The name as- 

 signed to a genus of fossil herbi- 

 vorous marsupials, found in the 

 Purbeck series. 



PLAGIO'STOMA. (from TrXa^os, oblique, 

 and a-rofia, a mouth, Gr.) A genus 

 of bivalve shells, first named by 

 LI why d and established by Mr. 

 Sowerby in his mineral conchology 

 for a generic form presumed to be 

 known only in a fossil state. 

 Certain differences of aspect in- 

 duced Mr. Sowerby to divide them 



