SHELL, a substance of a stony hard ness, composed of carbonate of lime, va riously comb ned with animal gluten, and serving for the coverings and habitations of different animals, mostly of the order of Mollusca; allowing of the occasional protrusion of part of their naked body. The various forms, the beautiful colours, and the high polish, which shells possess, have long rendered them objects of re search to the citrons naturalist ; many of them possessing these properties in a high degree; and being from their rarity purchased at very high prices.
The vast variety of forms which shells possess not only renders arrangement more necessary, but at the same time oc casions the formation of that arrangement the more difficult. Dissimilarity in size and colour, as well as in form, which oc cur in shells of the same species, at dif ferent ages, adds much to this difficulty. External characters, however, sufficient ly marking specific differences, may al most always he discovered in shells, at every period of their existence ; and va rious attempts have hence been made to dispose them in such a methodical ar rangement, as should place each of them in its proper place ; and should thereby occasion them to be more readily recol lected.
Lister, Langius, and other early writers on testaceology, have proposed different methodical arrangements of shells, which, from their imperfections, have been en tirely laid aside, and do not, therefore, require to be here noticed. Indeed it appears to be unnecessary to go further back than to the arran:!ement of the ce lebrated Linnxus, which seems, in its turn, to be yielding to later systems, ren dered more perfect by the constant ac cession of new and illustrative specimens, and the consequent increase of know. ledge. But previous to considering the proper method of arrangement, and to examine into even the generic differen ces, it will be proper to ascertain the cha racters which belong to shells in general, and to determine the precise terms by which they can be best expressed.
The division into univalves, bivalves, and multivalves, is so clearly pointed out by nature, that it must be adopted in eve ry well methodized arrangement, and may therefore be safely admitted in these pre liminary observations. Univalve shells are discoidal, when the spiral is formed on a horizontal line, so that a section, made in that line, would divide the shell into two nearly equal parts; fusiform, when it bul ges in the middle, and has the two extre mities of nearly equal length : turbinated, when the belly of the shell is very large and tumid, compared with the spire which is given off from its centre; and turricu lated, when the turns of the spire gradu ally increasing, form an elongated cone.
The parts of univalve shells are, the back, which is the external bulging part of the turn opposite to the opening : the belly, the tumid part of the last turn, forming the right or outer lip ; that being termed the left which is formed on the co lumells, and which Linnxus distinguish ed as the columellar ridge. The spire is formed on the superior part of the shell, the turns of which are formed round the coltimella, or little central column, reach ing from the opening of the shell to the summit of the spire. These convolutions are numbered by reckoning that as one, which reaches from the external to the inner lip, and in the same manner to the summit ; the line by which they are unit ed being termed a suture. The spire it self is either pointed, obtuse, truncated, flattened, concave, coilvex, straight, ob lique, or pyramidal. Its turnings are co ronated, (beset with spines or tubercles,) beaded, carinated, or canaliculated. The suture is crenelated, double, projecting, or effaced. The shell is said to be perfo rated when a small umbilical cavity exists in the columella at the base of the shell, in the axis round which the spire is re volved ; and imperforate, when there is here no cavity nor umbilicus. Besides being either umbilicated or not, the co lumella may be flattened, truncated, cau dated, canaliculated, spiral, and plaited, as in the Volutes. The umbilicus also may be slit, canaliculated, consolidated, crenu lated, or dcntated : it may also have a su peradded cullus. The opening varies much in different shells ; and where it narrows it acquires the name of throat ; it terminates in a canal. The lip is eared, digitated, slated, grooved, slit, or wrin kled ; the form of the opening depending much on that of the lip ; it hence becom ing angular, round, semicircular, longitu dinal, transverse, linear, gaping, corn. pressed, or reversed. The openings of shells are sometimes closed by a shelly or cartilaginous plano-convex body, termed operculum, which is disposed between the two lips, and fitted to the form of the opening. The flat surface of this body is marked by a spiral line.