Page:The American Cyclopædia (1879) Volume V.djvu/205

 CONCHOLOGY 201 ranged in the successive layers, separates into plates parallel with its surface an example of laminated structure. In the firachiopoda the structure is so peculiar that it may be detected in the smallest fragment of shell ; it consists of elongated and curved cells matted together, and often perforated by circular holes arranged in quincunx order. Colors, however beauti- fully exhibited upon the surfaces of shells, are to them no more distinctive features than to the minerals 3,nd flowers upon which they are also brilliantly displayed. They are most richly developed upon those surfaces most exposed to the light, and in the class of shells that inhabit shallow waters. In some instances they have proved as permanent as the shell itself, being preserved in a few of the fossils of various geo- logical formations, even as old as those of the Devonian period. The color is usually limited to the surface beneath the epidermis, which is the membrane of animal matter coating the entire shell, and protecting it from the action of destructive chemical agents. But some of the shells of porcellanous texture present dif- ferent colors in adjoining layers, a peculiarity which adapts them for the carved work in colors described in the article CAMEO. The shells of a single piece appear in various forms ; most of them are in spiral convolutions, which form logarithmic curves. In some the spire is wound around an axis called a columella, which is entirely concealed within the whorls; in others these wind about without coming in contact, the shell then resembling a corkscrew. Some are smooth conical bodies, terminated at the base of the cone by an insignificant spire, and with a long aperture on one side. In some the spiral form is very prominent in the young individuals, and is afterward entire- ly concealed under the successive layers of growth. In a few instances the convolutions are so flattened that no conical form is produced. The argonaut is an example of the last, a shell almost as delicate as pa- per, and so light as to be no im- pediment to the animal's rising to the surface by means of its dis- tended air cells. The haliotis (sea- ear) is also an ex- ample of a very flat and in this instance obscure spire. The aper- ture of this shell is open, resembling the ear in form, whence its name. To the animal the shell is a covering shield, which protects him as he draws it down, and holds by his broad foot firmly to the rock. Many other UNIVALVES. 1. Navicella. 2. Um- brella. 3. Patella. mollusca similarly provided are distinguished by shells shaped like a shield, as umbrella; or like a boat, as namcella ; or of more conical form, as the patella or limpet. The univalves take also the form of a tube open at each end. In describing shells, they are supposed to be in the position which the ani- mals assume when in motion and progress- ing from the observer. The aperture of the univalves is downward, and the spire points backward and upward ; the two sides are dis- tinguished as right and left. The whorls wind obliquely from the apex- of the spire from left to right in the greater number of species; those in which the direction is reversed are known as reverse or sinistral shells. Individ- ual exceptions occur in the species of each group. Bivalve shells of the class conchifera, in which all are included that breathe by two pairs of gills, take when in motion a position with the hinge of the valves upward. Two prom- BIVALVB SHELL. a, a, bosses; 6, &, hinges; c, principal teeth ; d, d, lateral teeth ; e, e, muscular impressions ; /, pallial impression ; g, g, sides of the shell ; A, ligament ; inences called the umbones are observed, one on each valve bending toward each other ; the beak or apex of each of these corresponds to the apex of the spire of the univalves. It often inclines toward one end of the shell; this is always the anterior extremity, and the opposite the posterior. The latter is also marked in many bivalves by a notch or sinus in the pal- lial impression upon the inner surface of the shells, always opening, when it occurs, toward this extremity. The pallial impression is the mark of the margin of the mantle where it was attached to the shell. The position of some bivalves, as the large scallops, oysters, &c., is indicated only by the large muscular impres- sion which is always on the posterior end unless it be between the umbones. The bi- valve being then placed in its natural position for progressing from the observer, the valves are designated, one as the right, the other as the left. If the two be of the same size and
 * , ventral edge ; o, front edge ; r, umbo.