My chancing on that exhibit was the impetus for this column. Ray Gildner has written a Java Applet, which is set up here with his permission, implementing Raup's simplified coiling model. W affects the rate at which the diameter of the tube grows. Higher values mean opening of the shell becomes wider with each rotation. D affects the distance of the center of the tube from the axis of coiling.
Higher values mean tube of the shell forms further from the coiling axis.
T affects the distance of the center of the tube from the previous rotation along the axis of rotation. Higher values mean the shell is taller. Once you've made your selections from the sliders, click the Draw button or click the image of the shell to redraw the image. To more clearly see how the different parameters affect the shell's shape, change one parameter, click Draw, and look for the differences. Terminal growth and unchanging form 2.