The bullids possess a very thick shell that is generally ovate but can tend to be cylindrical and always has a sunken spire. The shell is sombre coloured and mottled with various shades of brown. The animal can withdraw completely into the shell but there is no operculum to seal the large aperture.
The front corners of the head shield are developed into rolled tentacles and these short tentacles act as siphons. Posteriorly the head shield is developed into a pair of elongate, triangular lobes that just overlap the front of the shell.
The eye spots are obvious on the dorsal surface of the head shield.
The large foot is longer than the shell and provides small lateral parapodia folding over the neck and aperture of the shell. The gill is not visible externally.
Bullids are strictly nocturnal, emerging at night from the substrate to feed on algae.