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C L I 



In Cuvier's arrangement the cirrhopoda 

 form the sixth class of Mollusca. Lin- 

 nseus comprised them all in one genus 

 Lepas ; they have since been divided into 

 two, and again, by others, subdivided. 

 CI'RRUS. (cirrus, Lat. ) A genus of 

 fossil spiral shells of the chalk deposit. 

 This genus bears great resemblance to 

 trochus, from which, however, it may be 

 distinguished by its deep funnel-shaped 

 umbilicus. Two species of cirrus, 

 namely, cirrus depressus and cirrus per- 

 spectivus, are figured in Dr. Man tell 's 

 " Geology of the South-east of Eng- 

 land." 



CLATHRA'RIA LYE'LLII. A fossil plant 

 discovered by Dr. Mantell, and thus 

 named by him in honour of Charles Lyell, 

 Esq. The following description is ex- 

 tracted from Dr. Mantell's works. The 

 Clathraria Lyellii bears an analogy to the 

 yucca, and dracsena or dragon-blood 

 plant. Stems, with the markings of the 

 bases of the leaves, point out the relation 

 of this vegetable to the arborescent 

 ferns, while its internal structure is 

 essentially different. The clathraria has 

 only been found in the quarries in Fel- 

 gate Forest. This vegetable appears to 

 have possessed a thick epidermis, or false 

 bark, formed by the union of the bases of 

 the leaves, and covered externally with 

 distinct rhomboidal scales, each scale 

 being surrounded by an elevated ridge. 

 The form of the leaves is not positively 

 known, although, from some imperfect 

 traces on the stone in a specimen bear- 

 ing the impressions of the cicatrices of 

 the bases of the leaves, there is reason to 

 conclude that they were of a lineari -lance- 

 olate form. The axis, or interior part of 

 the trunk, originally enclosed by the 

 bark, occurs in the state of solid subcy- 

 lindrical blocks of sandstone, attenuated 

 at their base, the surfaces of which are 

 marked with longitudinal interrupted 

 ridges, and, in some instances, are deeply 

 imbricated ; they are generally of a dark- 

 brown colour. 



CLAVAGE'LLA. A genus of bivalves, of 

 which only one species has been found 

 recent in the Sicilian seas. It has two 

 irregular, flattish valves, one of which is 

 clasped by the tube, the other being left 

 free. Mr. Sowerby observes, " The shells 

 composing this genus are found in stones, 

 madrepores, &c. and appear to form the 

 connecting link between Aspergillum, 

 which has both valves cemented into the 

 tube, and Fistulana which has both 

 free." 



CLA'VICLE. (clavicula, Lat. clavicule, Fr. 

 clavicula, It.) The collar-bone. 



CLAY. When clay is quite pure and un- 

 mixed (and in this state it is one of the 



rarest substances in the mineral king- 

 dom) it is termed alumina, but under the 

 term clay is comprehended an extensive 

 class of compounds, of which silex is a 

 principal constituent. Clay, then, may be 

 denned an unctuous and tenacious earth, 

 capable of being moulded into form. 

 Clays are firmly coherent, weighty, com- 

 pact and hard when dry, but stiff, viscid, 

 and ductile when moist ; being smooth 

 also and unctuous to the touch. Besides 

 alumina and silica, clays often contain 

 carbonate of lime, magnesia, barytes, 

 oxide of iron, &c. When clay is breathed 

 on, it yields a peculiar smell"; it has also 

 a strong affinity for moisture, which is 

 shewn by its sticking to the tongue, 

 when applied to it. 



CLAY-SLATE. (The argillite of Kirwan.) 

 An indurated clay or shale, common to 

 the fossiliferous and metamorphic series. 

 Clay- slate is opaque, of various shades 

 of colour, and of different degrees of 

 hardness, but easily scratched by iron. 

 It is composed of about fifty per cent, of 

 silex, twenty-five of alumine, and ten or 

 twelve of oxide of iron. Some varieties 

 are used as whetstones. The common 

 hone is a variety of slate containing a 

 smaller proportion of alumine and some 

 lime. 



CLA'YSTONE. An earthy stone resembling 

 indurated clay, and generally of a colour 

 approaching to purple ; it is a variety of 

 prismatic felspar. 



CLEA'VAGE. A peculiar fracture, im- 

 pressed by nature, which is sometimes 

 mistaken for stratification. This is pret- 

 tily described by Dr. Mantell: "If I 

 take a flint and break it at random, it 

 still preserves a conchoidal fracture, a 

 sharp cutting edge ; and subdivide it as 

 I may, it still retains the same character. 

 If I shiver to pieces calcareous spar, 

 every fragment presents, more or less dis- 

 tinctly, a rhomboidal form ; so true is 

 the remark, that we cannot break a stone 

 but in one of nature's joinings." 



The regular partings or cleavages in 

 many slate rocks which intersect the beds, 

 nearly at right angles to their dip or in- 

 clination, have often been mistaken for 

 strata seams, and have led geologists of 

 some eminence to draw very erroneous 

 inferences. 



CLEA'VELAXDITE. A mineral, to which 

 this name has been given after Professor 

 Cleaveland ; it has been also called 

 albite. 



CLEFT. A space made by the separation 

 of parts ; in rocks, a crack or crevice. 

 LEFT. Divided ; cloven. 



CLI'MBING. In botany, plants are called 

 climbing, when they mount up other 

 bodies (namely, walls, trees, &c.), and 



