Snails have an extraordinary power of re-producing any part which may be injured or cut off, even to the extent of the whole head, as has been observed to be the case; the reparation of injury done to the shell they can effect easily, as can all testaceous mollusks. Respecting the construction of the shell it may be observed that it is produced in the thickness of the mantle, or cloak-like covering which envelopes the body of the animal; the formation commences at the small end or spire, and gradually goes on, whorl upon whorl, as the still widening circles which give the ridgy appearance to most univalves, are called. _Columella_, or _pillar_, is the name given to the spire on which the cones are rolled; this is sometimes solid, and sometimes hollow; when the latter, the open end is called the _Umbilicus_, meaning the navel or centre. The opening at the bottom, from which the animal issues, is the last portion finished, and this is called the _aperture_, a Latin word adopted into the English dictionary. Some of these _Helices_ are wide and flat, even hollow and cup-like, with the whorls rising above the pillar—these are called _discoid_ shells; others which are long and narrow, with projecting spires, are termed _turbinated_ shells; the former being more or less flat or disk-like, the latter twisted, whirling, like a spinning-top, from the Latin _turbo_—a whirling, a turning round. [Illustration: DISCOID.] [Illustration: TURBINATED.] If we take a Common Snail, and plunge it into boiling water, which will instantly kill it, so that it can be removed from the shell, we shall find the whole of that part of the body which was lodged in the upper whorls, or spiral part of the shell, is covered with a thin membrane or skin; this is called the _mantle_, and that portion of it which corresponds with what we may consider as the back of the mollusk, and which is considerably thickened, is termed the _collar_; here are situated the glands, which secrete the colouring and other matter of which the shell is mainly composed; although the substance called nacre, or mother-of-pearl, is secreted in the thinner part of the mantle; it is however from the collar that the growth or increase of the shell proceeds. It is in accordance with certain variations in the shape and disposition of this mantle and collar, that shells assume such very different shapes. Sometimes the whorls or spiral ridges, are projected or thrown far out, and this produces the _turbinated_ shell. Sometimes they scarcely rise above each other, but rather spread towards the sides, and then we have the _discoid_ shape. Generally speaking, the whorls of a shell take a direction from left to right, but occasionally an opposite one; they are then called sinistral, or left-handed shells: such are not common. If one of the twisted shells be divided length-ways, it will be seen that the inside of the whorls wind in an ascending direction,