Shells minute to small, sorted in two subfamilies of extremely different shapes, aggregated in the same family due to common anatomical characteristics. Members of the subfamily Addisoniinae are limpet-shaped and live inside Euselachii eggs, while the Helicopeltinae have snail-like shells and live on « detrital cephalopod beaks » (Marshall, 1996).
 

« Shell ovate, subconical, strongly asymmetrical, porcellanous, thin; with a blunt apex curved backward, downward, and to the left, without an epidermis; with an unthickened, simple, entire margin; pedal muscular impression horseshoe-shaped, interrupted in front. » – W. H. Dall: “On certain limpets and chitons from the deep waters off the eastern coast of the United States”, Proceedings of the United States National Museum vol.4, Washington 1881, p.405.