A bulla (plural "bullae") was round, ovoid, cone-shaped, or heart-shaped apotropaic amulet given to male infants in ancient Rome on their ninth day of life after being born. Bullae were worn on a chain around the child's neck, and they were sometimes covered in gold foil. Bullae were hollow like lockets but not intended to be open, containing small symbolic protective items (chiefly phallic symbols). One might speculate that the rattling sound of these items served both to entertain the infant and to give an audible indication of where he was, much like putting a bell on a cat's collar. The word bulla is Latin for "blob" or "bubble." The bulla was worn by any freeborn Roman boy until he reached the age of twenty-five years and became a full citizen of the Roman Empire. At this time he would exchange his toga praetexta, the apotropaic uniform of freeborn legal minors, for the toga virilis, the uniform worn by male full citizens on formal occasions. The bulla would be placed on the household lararium, a shrine to the household protective deities and ancestral spirits, and it would remain there for the rest of his life, only to be removed from the lararium under two conditions: either to be interred with him among his grave goods, or if he should accomplish a military triumph, it might be worn by him as part of his ceremonial regalia in the triumphal procession.

Similar amulets have been found in Ireland, made of clay or base metals (and, like in Rome, sometimes covered in gold foil) and dating to the Late Bronze Age, between 1150 and 750 BCE.

Roman female children had a corresponding crescent-shaped amulet called a lunula, "little moon," which was conventionally gifted to the child on the day of her birth, or on any subsequent birthday, rather than specifically the ninth day as a boy's bulla. On the eve of her wedding, the lunula and all her childhood toys and apparel would be placed on the lararium or dedicated as an offering either to Venus or to Fortuna Virginalis, the aspect of the goddess Fortuna believed to protect girls before their marriage.

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