The aperture is generally large, varying from about two-fifths to one-third the height of the shell, often within the same population. Usually the lip is at least somewhat expanded, and in forms such as "A. reflexilabris" Schepman and "A. winteri" (Pfeiffer) var. "inauris" Fulton, the lip can only be called flaring. In "A. perversus" (Linnaeus) and most other thick-shelled species, the lip is internally thickened, forming a "roll" in its expansion, and has a very heavy parietal callus. In thin-shelled species, the lip is usually a simple reflected edge. The umbilical area can be partially open, nearly closed, or sealed. This feature sometimes provides a useful criterion for specific identification. The angle of the parietal wall varies, but no precise information on this has been compiled.