Differ, in Conchology; a genus belonging to the order of vernes testacea. It is an animal of the snail kind: the shell consists of one valve, convoluted, and without any pricks; the aperture is narrowish, oblong, longitudinal, and entire at the base; the columella is smooth and oblique. There are 23 species; four of them found in the British seas; the rest chiefly natives of the Asiatic and Atlantic oceans. See Conchology Index.

Bullæ, in antiquity, a kind of ornaments much in use among the ancient Romans. Mr Whittaker* History of Manchester is of opinion that they were originally formed of leather among all ranks of people; and it is certain that they continued so to the last among the commonalty. He also imagines, that at first the bulla was intended as an amulet rather than an ornament; as a proof of which he tells us that the bullæ were frequently impressed with the figure of the sexual parts. It is universally asserted by the critics, that the bullæ were made hollow for the reception of an amulet; but this Mr Whittaker contradicts from the figure of a golden one lately found at Manchester, which had no aperture whereby an amulet could have been introduced.—Pliny refers the original of the bulla to the elder Tarquin, who gave one with the pretexta to his son, because at the age of 14 he had with his own hand killed an enemy; and in imitation of him it was afterwards assumed by other patricians. Others affirm that the bulla was given by that king to the sons of all the patricians who had borne civil offices. Lastly, others allege, that Romulus first introduced the bulla, and gave it to Tullus Hostilius, the first child born of the rape of the Sabines.—As to the form of the bulla, Mr Whittaker informs us that they were originally made in the shape of hearts; but they did not always retain the form of a heart, any more than they were always made of leather. As the wealth of the state and the riches of individuals increased, the young patrician distinguished himself by a bulla of gold, while the common people wore the amulet of their ancestors. The figure of the heart then became so generally round, some even having the impression of an heart upon them, that there are not many of the original form to be found in the cabinets of the curious. The form is naturally varied from a complete circle to that of a segment; and this was the shape of the above-mentioned bulla found at Manchester. When the youth arrived at 15 years of age, they hung up their bullæ about the necks of their gods lares. We are further informed, that the bullæ were not only hung about the necks of young men, but of horses also. We may add, that bullæ were sometimes allowed to statues; whence the phrase statua bullæ.