mississippiensis Pilsbry, 1886 Pyrgulopsis mississippiensis Pilsbry, 1886 Marstonia scalariformis, previously known as Pyrgulopsis scalariformis, common name the moss pyrg, is a species of freshwater snail in the family Hydrobiidae.

The shell is carinate in its entire length on the lower edge of the whorls.

[5] The aperture is small, ovate, but slightly connected with the last whorl.

[4] Distribution of Marstonia scalariformis include Illinois,[1][6] Alabama,[1][7] Iowa, and Missouri, all within the Mississippi River System.

[1] This species was abundant on its type locality on the Tazewell shore of the Illinois River in 1880s.