If the tips of the triangles are raised, the threads form a surface called a hyperbolic paraboloid. The model came to the Smithsonian from the Department of Mathematics at the University of Michigan.

Two distinct borehole geometries cylindrical and paraboloidal, are reported in brachiopod shells from the Edwardsville Formation (Lower Mississippian) of southern Indiana. The cylindrical boreholes ...

Henry Burchard Fine and Henry Dallas Thompson, Coordinate Geometry, New York: Macmillan Company, 1931, p. 243-244. An example of a related Brill model, with more lines of intersection shown on it, is ...