shells.  They vary in colour from the palest straw to the deepest purple.  Some of them are exceedingly beautiful and valuable, and they are nearly all more or less rare, being found chiefly in small fragments of ancient pavements.  Their substance is formed of the shells of the common oyster in bluish gray and black particles on a white ground, as in the Lumachella d’ Egitto; of the cardium or cockle, assuming a lighter or deeper shade of yellow, as in the Lumachella d’ Astracane; of the ammonite, as in the L. Corno d’ Ammone; of the Anomia ampulla in the L. occhio di Pavone, so called from the circular form of the fossils whichever way the section is made; of encrinites, belemnites, and starfish, showing white or red on a violet ground, as in the L. pavonazza; and “of broken shells, hardly discernible, together with very shining and saccharoid particles of carbonate of lime,” as in the Marmor Schiston of the ancients—­the brocatello antico of the Italians, so named from its various shades of yellow and purple, resembling silk brocade.  The most important specimens of Lumachella marbles are the pair of very fine large columns of L. rosea on the ground-floor of the Schiarra Palace, the balustrade of the high altar of St. Andrea della Valle, two columns in the garden of the Corsini Palace of L. d’ Astracane, and a pair of large pillars which support one of the arches of the Vatican Library, formed of L. occhio di pavone.  Specimens of brocatello may be found in several churches and palaces, forming mouldings, sheathings, and pedestals.

The most interesting of the Lumachella marbles is the bianca antica, the Marmor Megarense of the ancients, composed of shells so small as to be scarcely discernible, and so closely compacted that the substance takes a good polish.  The well-known Column of Trajan—­the first monument (columna cochlaea) of this description ever raised in Rome, and far superior to the Antonine Column—­is composed of Lumachella marble from Megara.  It presents, in twenty-three spiral bands of bas-reliefs, winding round thirty-four blocks of stone, the history of the victories of Trajan over the Dacians, and, without reckoning horses, implements of war, and walls of cities, is said to consist of no less than two thousand five hundred figures, each about two feet two inches high.  It is a strikingly suggestive thought, that this majestic pillar—­which produced so deep an impression upon the minds of posterity that, according to the beautiful legend, Pope Gregory the Great was moved to supplicate, by means of masses in several of the Roman churches, for the liberation of him whom it commemorated from purgatory—­should be composed of the relics of sea-shells.

    “Memorial pillar! ’mid the wreck of Time,
    Preserve thy charge with confidence sublime,”