P H 



[S29 ] 



ORC 



green. It contains greenish -white 

 crystals of felspar, which on the 

 polished surface often shew them- 

 selves in parallelograms, and are 

 sometimes cruciform. It occurs 

 massive and disseminated. Lustre 

 glistening and resinous. Fracture 

 conchoidal, and often splintery. 



OPHITJ'EA. A genus of fossil radi- 

 aria, of the order Ophiuridse, found 

 in the lias of Yorkshire and in the 

 inferior oolite sands near Bridport; 

 in the oolite of Germany, in the 

 muschel kalk, in the red sandstone 

 group, in the upper lias, and in 

 the upper chalk. 



OPHUI'BIDJE. The third order of the 

 class Echinodermata. 



OPI'STHOBRANCHIATA. The sixth order 

 of the class Cephalophora, com- 

 prising Tornatella, Bulla, Doris, 

 Aplysia, &c. 



OPO'SSUM. A genus of quadrupeds 

 belonging to the order Marsupialia. 

 The opossums are peculiar to 

 America, and are remarkable for 

 their number of teeth, amounting 

 in all to fifty. They approach the 

 quadrumanes, by having the thumb 

 of their hind foot opposed to the 

 fingers, whence they have been 

 called pedimanes : the thumb is 

 not armed with a nail. 



OBBI'CULA. (from orlis, an orb, Lat.) 

 A genus of bivalve shells, fossil 

 and recent, belonging to the family 

 Brachiopoda. The orbicula is a 

 very small inequivalved flat bi- 

 valve; the lower valve very thin, 

 and adherent to other bodies. 



OBBI'CFLAB. (orbiculaire, Fr. orbico- 

 Idre, It.) Spherical; circular; 

 roundish and flat. In botany, 

 leaves are so called when their 

 length and breadth are equal, and 

 their form nearly circular. 



O'BBIT. (orlita, Lat. orlite, Fr. drbita, 

 It.) The line described by the 

 revolution of a planet ; the path of 

 a planet, or of a comet. The mean 

 distance of a planet from the sun 

 is equal to half the major axis of 



its orbit. A planet moves in its 

 elliptical orbit with a velocity 

 varying every instant, in conse- 

 quence of two forces, one tending 

 to the centre of the sun, and the 

 other in the direction of a tangent 

 to its orbit, arising from the primi- 

 tive impulse, given at the time 

 when it was launched into space. 



OEBTJLI'TES. A genus of foraminated 

 polypifers, differing from lunulites 

 in having pores on both sides. Mr. 

 Parkinson thus describes the genus: 

 *' a free, circular, stony polypifer; 

 flattish ; pores on both sides, or at 

 the margin ; resembling nummu- 

 lites. Set with minute pores, 

 sometimes scarcely visible, regularly 

 disposed, but not in spiral order." 

 The species are nearly all fossil 

 but some are recent." 



OBCHI'DE^. (from o'/>X ts > Gr. orchis, 

 Lat.) An order of monocotyle- 

 donous plants. Perianth superior, 

 sepals three, usually coloured, the 

 odd one uppermost, from the twist- 

 ing of the ovarium ; petals three, 

 usually coloured, of which two are 

 the uppermost, while the third, 

 called the labellum, is usually lobed, 

 and differs in figure, colour, or size, 

 from the other two, and is often 

 spurred; stamens three, united in 

 a central column, the two lateral 

 generally abortive, the central one 

 perfect; anther persistent or decid- 

 uous; pollen either powdery or 

 cohering in granular or waxy 

 masses; ovarium one-celled, with 

 three parietal placenta ; style 

 forming part of the column of the 

 stamens ; stigma a viscid space in 

 front of the column ; fruit usually 

 a capsule, dehiscing by three valves, 

 sometimes baccate ; seeds numer- 

 ous, testa loose and reticulated, no 

 albumen, embryo a solid undivided 

 fleshy mass; herbaceous plants, 

 either stemless, or forming a kind 

 of tuber above ground; or some- 

 times with a true stem; leaves 

 fiimple and entire, sometimes artic- 



