What is "staircase shell"

Wentletrap \Wen"tle*trap`\, n. [D. wenteltrap a winding staircase; cf. G. wendeltreppe.] [Obs.] Any one of numerous species of elegant, usually white, marine shells of the genus Scalaria, especially Scalaria pretiosa, which was formerly highly valued; -- called also staircase shell. See Scalaria.