"The _columella_, or _pillar_, extends through the centre of the shell withinside. The _Buccinum_ and _Voluta_ both show the columella. (Plate 3.) [Sidenote: TERMS--UNIVALVES.] "The _pillar-lip_ of the aperture, or columella margin, is on the left-hand side of the shell; the _outer_ lip on the right-hand. Occasionally this order is reversed, but not commonly. "The _operculum_, or lid, in univalves is that part which fits exactly into the aperture, and incloses the animal; it serves as a door to the shell. (Plate 3.) The operculum is either horny, like that of the periwincle, or of a harder substance, like the shell itself. "The _umbilicus_ is a circular hole in the body of the shell. This perforation produces a very curious effect when it is very large. When the umbilicus is wanting, the shell is called _imperforate_. "If the spire is truncated, it is _decollated_; if it is surrounded with spires, it is _coronated_, or crowned. * * * * * "Second section, without a regular spire. The _teeth_ in univalve shells, as the cowry, are ridges upon the aperture. (Plate 1.) In _Voluta_ they are regular folds or plaits upon the columella. "A _fissure_ is a notch or slit, as in _Patella fissura_. (Plate 3.) "Some shells of this section are internally lipped, as _Patella equestris_; (_Calyptræa_ of other authors, Plate 3;) some are chambered, as the slipper-limpet; some are cap-shaped, having the apex much curved--these are the cap-limpets. [Sidenote: TERMS--UNIVALVES.] "The _vertex_ in _Patella_ is as the superincumbent part of the shell. "The _epidermis_ is the outward skin that covers the surface of some shells. "_Fauces_ are narrow entrances, as at each end of the cowry. "_Tubercles_ are protuberances, or knobs, on the surfaces of shells. "_Striæ_ are raised or flat lines upon the surfaces of shells. When the surface is marked with lines longitudinally and transversely, it is _decussated_. "_Sulci_ are furrows. "_Fornicated_ signifies arched, greatly excavated. "_Umbo_ is the swelling part near the beak of bivalve shells; the same as _boss_. "_Longitudinal_, running nearly the whole length of the shell in univalves. "_Concentric_, having the same centre. "_Convolute_, when the exterior whorls spirally involve the interior. "_Varices_, longitudinal, gibbous sutures formed in the shell, at certain distances on the whorls. "_Carinated_, having the form of the keel of a boat." CHAPTER III. [Sidenote: SYSTEM OF LINNÆUS.] "As you both assure me," said Mr. Elliot, "that you do not fail to make yourselves familiar with the _nomenclature_, or terms, used in conchology, I shall proceed this morning to the arrangement of Linnæus. "This system is established upon the _outward_ appearances, or external characters, of the covering bestowed by nature upon the animal, not upon the form of the animal itself.