A wind instrument, anciently made of horn, but afterwards of brass. Like the tuba, it differed from the tibia in being a larger and more powerful instrument, and from the tuba itself, in being curved nearly in the shape of a C, with a cross-piece to steady the instrument for the convenience of the performer. It had no stopples or plugs to adjust the scale to any particular mode; the entire series of notes was produced without keys or holes, by the modification of the breath and of the lips at the mouth-piece. The classicum, which originally meant a signal, rather than the musical instrument which gave the signal, was usually sounded with the cornu. - Smith, 1873