BULLA, or DIPPER, in zoology, a genus belonging to the order of vermes testaceæ. It is an animal of the snail-kind: the shell consists of one valve, convoluted, and without any prickles; 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 †.