"Amphidromus" has an elongate-conic or ovate-conic helicoid shell of 5 to 8 whorls. Flowers are solitary or in inflorescences in a raceme or a helicoid or dichasial cyme. The shell contains 7 whorls, including a small smooth protoconch of two helicoid whorls. The meridian of a helicoid is a line which intersects the axis orthogonally. Wrapped around the grana are helicoid stromal thylakoids, also known as frets or lamellar thylakoids. The protoconch is helicoid, with a very small initial, and rapidly increasing subsequent whorls. The helicoid is shaped like Archimedes screw, but extends infinitely in all directions. Stromal thylakoids are helicoid sheets that spiral around grana. A closed ruled generalized helicoid has a profile line that intersects the axis. "Hedleyella falconeri" has a helicoid (spiral) shell and grows to a height of about . However, not every ruled surface is developable; the helicoid provides a counterexample. The shape may also be subglobose, turbinate or helicoid. If the profile curve is a line one gets a "ruled generalized helicoid". Classic examples include the catenoid, helicoid and Enneper surface. The helicoid is homeomorphic to the plane formula 5.