The top shell family is very large, with hundreds of species and many genera. Most show an iridescent interior, numerous whorls, and a horny operculum. Varying shell characteristics have led to specification of several distinctive subfamilies. For example, in the Monodontinae, the base is quite flattened, giving the appearance of an inverted top. In the Calliostominae, a large, rounded aperture opens more to the side and makes the base markedly less flat. In the subfamily, Angarinae, the final whorls become separated rather than remaining sutured, leading to a species rather atypical in appearance.