O X F 



C 192 ] 



PAL 



pistil, are called ovula, or ovules ; when ; 

 perfected they become the seeds of the 

 plant. The ovule is generally attached 

 to the placenta of the ovarium by a very 

 small stalk. 



O'xFORn CLAY. Called also clunch clay. 

 A bed of dark blue clay, sometimes 

 nearly two hundred feet in thickness, in- 

 terposed between the lower and the mid- 

 dle oolites. One species of ichthyosau- 

 rus, distinct from the species occurring in 

 the lias, has been found in this deposit. 



OXIDA'TION. That process by which me- 

 tals, and other substances, are converted 

 into oxides by their combination, with 

 oxygen. 



O'XIDE. A substance combined with oxy- 

 gen, without being in the state of an 

 acid. 



O'XIDIZED. Converted into an oxide, by 

 combination with oxygen. 



O'XYGEN. (from ot>c, acid, and ytwdb), 

 to produce, Gr.) So called from its 

 property of forming acids. One of the 

 fifty-five simple or elementary substances, 

 and one of the five which exist as gas. 

 So generally does oxygen enter into com. 

 bination with metallic and non-metallic 



bodies, and in such large proportions, 

 that it has been computed that one-half of 

 the ponderable matter of the globe is 

 composed of oxygen gas. Oxygen con- 

 stitutes about one-20th per cent, of the 

 volume of the atmosphere ; it forms a 

 third part, by measure, of the gases com- 

 posing pure water ; and is locked up to 

 an immense amount in the various rocks, 

 which are little else than a mass of oxi- 

 dized substances. Plants give out oxy- 

 gen, animals absorb it. It is to Dr. 

 Priestley we owe the knowledge of the 

 former of these two facts ; and he it was 

 who first discovered oxygen, in 1774. 

 Oxygen has neither taste nor smell. It 

 is a trifle heavier than atmospheric air, 

 100 inches weighing 33'88 grains. 



OXYGENA'TION. "This word," says Dr. 

 Ure, (t is often used for oxidation, and 

 frequently confounded with it : but it 

 differs, in being of more general import, 

 as every union with oxygen, whatever the 

 product may be, is an oxygenation ; but 

 oxidation takes place only when an oxide 

 is formed." Diet, of Chem. 



O'YSANITE. A name given by Lameth to 

 pyramidal titanium, or anatase. 



PACHYDE'RMATA. (from Trn\vQ, thick, 

 and Bepua, skin, Gr.) Thick-skinned 

 animals. The seventh order of the class 

 Mammalia, in Cuvier's arrangement. This 

 order Cuvier divided into two families, 

 namely, Proboscidiana, or those pachy- 

 dermatous animals which have tusks and 

 a proboscis, as the elephant and masto- 

 don ; and pachydermata ordinaria, in 

 which are included the hippopotamus, 

 anoplotherium, palseotherium, tapir, &c. 



Several genera of the order Pachyder- 

 mata have become extinct, their fossil 

 remains alone proving that such ever 

 existed. Amongst these are the masto- 

 don, the anoplotherium, the palaeothe- 

 rium, and the lophiodon. Of the exist- 

 ing genera of pachydermata, many species 

 which existed during the older and newer 

 pliocene periods also seem to have be- 

 come extinct. 



The pachydermata appear to be, as it 

 were, only the remnants of a very exten- 

 sive order, which formerly inhabited the 

 earth, but have now almost entirely dis- 

 appeared. They feed upon grass, but 

 they do not ruminate. They are, for the 

 most part, huge and unwieldy animals, 

 with thick integuments ; solidity and 

 strength appearing to be the objects 

 chiefly regarded in their construction. 



PACHYDE'RMATOUS. Thick-skinned ; be- 

 longing to the order Pachydermata. 



PA'DDLE. The swimming apparatus of the 

 chelonian reptiles, and of the marine sau- 

 rians, has obtained the name of paddles. 



PJSCILO'PODA. (from TTOIKJAOC, and Trove, 

 Gr. various footed.) The second order 

 of the class Crustacea ; it comprises two 

 families, Xysophura and Siphonostoma. 



PA'LATAL. Pertaining to the palate, or 

 roof of the mouth. 



PA'LATE. (palatum, Lat. palais, Fr. pa- 

 lato, It.) The roof, or upper part of the 

 mouth. 



PAL^EO'LOGY. (from TraXatoc, ancient, and 

 Xoyoc, discourse, Gr.) The study of 

 ancient things. This word is commonly 

 written paleology. 



PAL^EONTO'LOGIST. (from palaeontology.} 

 One who studies, or is versed in, the 

 history of fossil plants and animals. 



PAL^EONTO'LOGY. (from TraXaioe, ancient, 

 oj/ra, beings, and Xoyoc, discourse, Gr. ) 

 The history of fossil plants and animals ; 

 that branch of natural history which treats 

 of fossil and extinct animals and plants. 



PAL^JOSAU'RUS. (from TraXntoc, ancient, 

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

 fossil saurians, now extinct, found in the 

 magnesian limestone. 



PAL^EOTHE'RIUM. (from TraXatoe, ancient, 



