E H 



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mentary canal.) A genus of fossil 

 crinoidea occrring in the mountain, 

 limestone. 



RHOMB. (/>o/*/3o<?, Gr. rhombus, Lat.) 

 In geometry, an oblique-angled 

 parallellograra, or quadrilateral 

 figure, whose sides are equal and 

 parallel, but the angles unequal, 

 two of the opposite sides being 

 obtuse, and two acute. 



RHO'MBOJD. (from popfios, a rhomb, 

 and eTdos, form, Gr.) A figure 

 which has its opposite sides equal 

 to one another, but all its sides are 

 not equal, nor are its angles right 

 angles. 



RHOMBOHE'DEON. A solid contained 

 by six plane surfaces, thfc opposite 

 planes being equal and similar 

 rhombs parallel to one another; 

 but all the planes are notnecessarily 

 equal or similar, nor are its angles 

 right angles. 



RHOMBOI'DAL. Having the form of a 

 rhomboid. 



RHOMB SPAE. Called also bitter spar, 

 a variety of magnesian carbonate 

 of lime. This variety occurs crys- 

 tallised, most of its crystals being 

 rhombs, sometimes truncated, and 

 sometimes with rounded edges. 

 Fracture foliated, the folia) having 

 a shining or splendent lustre, more 

 pearly than that of calcareous spar. 

 Specific gravity from 2*48 to 3. 

 It is generally of a greyish colour 

 to pale yellow, or yellowish brown. 

 It appears to pass by imperceptible 

 shades into dolomite. 



RIBB'ON JAS'PEE. A variety of jasper, 

 presenting green, red, and yellow co- 

 lours, of various shades. 



RIBBON AGATE. Called also Striped 

 Agate. A kind of agate which, 

 on being cut at right angles with 

 the layers of which it is composed, 

 presents alternate colours, some- 

 times straight, at others curved, 

 but always parallel with each other. 

 For a description see Agate. 



RI'MA. In conchology, the interstice 



between the valves, when the hy- 

 men is removed. 



BI'MULA. A genus of small patelliform 

 shells separated from Emarginula 

 on account of the position of its fissure 

 which approaches the margin but 

 does not reach it. The great and 

 inferior oolite of Gloucester have 

 species. Lycett. 



RI'NGENT. (from ringo, to grin, Lat.) 

 In botany, applied to a monopeta- 

 lous corolla, the border of which 

 is usually divided into two lips, 

 which gape like the mouth of an 

 animal. A corolla with two lips 

 is called bilabiate; when these 

 present an appearance like the 

 mouth of an animal, the corolla is 

 called rinyent. 



RINGI'CULA. A genus of fossil uni- 

 valves ; a species of which occurs 

 in the Suffolk Crag, and also at 

 Bordeaux, and which was formerly 

 attributed to Auricula. 



RI'SSOA. A genus of small turrited 

 univalve shells, placed by Sowerby 

 near to the ScalariaB. The aperture 

 is expanded anteriorly, and the 

 outer lip thickened. FOU? species 

 are known fossil in the great and 

 inferior oolite of Gloucestershire, 

 and in the great oolite at Ancliff. 

 Lycett. 



RO'ACH. A provincial term given to 

 the upper bed of the Portland oolite, 

 " it is," says Mantel], " a congeries 

 of casts of trigonise, pernae, terebrse, 

 ammonites, lucinaD, &c." 



ROCK, (roc and roche, Fr. rocca, It.) 

 Rocks, and the substances they 

 enclose, lie beneath the superficial 

 accumulations, and constitute the 

 crust of the earth. The term 

 " rocks" is apt to mislead begin- 

 ners ; for under this title geologists 

 rank clay, sand, coal, and chalk, as 

 well as limestone, granite, slate, 

 and basalt, and other hard and 

 solid masses, to which the use of 

 the term is generally restricted. 



The rocks of which the mineral 

 crust of the globe is composed are 



2 D 



