The Nautilidae are involute or slightly evolute and generally smooth with straight to sinuous sutures. The evolute of an involute is the original curve. Shells are variable in form, both ribbed, evolute forms and smooth, involute forms are included. The Prakriti as the insentient evolute, joins with Purusha which is sentient consciousness. The cycloid, epicycloids, and hypocycloids have the property that each is similar to its evolute. Ammonites belonging to this genus have evolute, moderately compressed shells. Equivalently, an evolute is the envelope of the normals to a curve. It is nearly identical in form to the evolute of an ellipse. The evolute of a curve is the locus of centers of curvature. The evolute of an astroid is an astroid twice as large. "Stephanoceras humphriesianum" has an evolute shell with well-developed ribbing and tubercles. "Stephanoceras" has an evolute shell, as characteristic of the family, with well-developed ribbing and tubercles. It differs from "Lioceratoides" by being more evolute and having thicker whorl section. Vertices of "C" correspond to singular points on its evolute. Their centers, i.e. the centers of curvature, form another curve, called the "evolute" of "C".