A conchological manual . ent method of describing the part inquestion. The columellar lip is sometimes detached entirelyfrom the body of the shell, as in Murex haustellum ; in othersit is decumbent, or lying over the last whorl, although quitedistinct, and in some cases, thickened, callous, or tumid. At the lower or anterior part, sometimes called the columella,there are in many instances flattened, laminated folds; theseare particularly conspicuous in the genera Cymba and Melo,where, being obliquely spiral and laminar, they are extremelyelegant, presenting to the eye graduated repetitions of
A conchological manual . ent method of describing the part inquestion. The columellar lip is sometimes detached entirelyfrom the body of the shell, as in Murex haustellum ; in othersit is decumbent, or lying over the last whorl, although quitedistinct, and in some cases, thickened, callous, or tumid. At the lower or anterior part, sometimes called the columella,there are in many instances flattened, laminated folds; theseare particularly conspicuous in the genera Cymba and Melo,where, being obliquely spiral and laminar, they are extremelyelegant, presenting to the eye graduated repetitions of the lineof beauty. In other cases, as in the Turbinellse, they aremore horizontal and thickened. INTRODUCTION. 29 In some cases the columella is swelled into a varicose mass;as in Oliva, Ancillaria, &c. ; it is then described as tumid orvaricose. It is sometimes tortuous^ and sometimes straight,and is susceptible of many variations, too minute and par-ticular to be described in this part of the work. Columellar Fig. 60, Melo, obliquely plaited; Gl, Turbinellus, horizon-tally plaited; 62, Ancillaiia, varicose, tortuous; 63, Na-tica, straight. OPERCULUM. The aperture of many species of shells remains constantlyopen ; but in a great number of species it is occasionally closed,whenever the animal is retracted within the shell, by a calca-reous or horny piece called the operculum. This must be dis-tinguished in the first instance from another kind of calcareouscovering, which in some univalve shells serves to close the aper-ture during a certain portion of the year. This piece, named theepiphragm, althougli hardened and shelly in appearance, is noreal part of the animal or of the shell ; being only a secretiontemporarily hardened, for the purpose of defending the animalfrom external influences during the hibernating or torpidseason, to be dissolved when that season is at an end. Onexamining this piece, it will be observed that it is not formed 30 INTRODUCTION. in regular laye
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Keywords: ., bookcentury1800, bookdecade1840, bookidconchol, booksubjectshells
