Fig. 5. Measurement and construction of the ridge. (A) Simultaneous top, side, front and oblique views are shown. Lines drawn on the different views indicate the extent of the ridge in these different views. In the front view, the lower line (yellow) indicates the spline curve defining the top of the odontophore not including the ridge, and the upper line (green) indicates the protrusion of the ridge above this curve. (B) A 100° arc of a circle whose radius is 1.23 radula stalk widths (RSW) (shown in grey) is a good fit to the radular surface below the region where the ridge occurs and is superimposed on the mid-sagittal outline of the odontophore extracted from the MRI to estimate the extent of the ridge. This curve is drawn in yellow on the side view of the radula in (A). The arc is continued posteriorly by a line tangential to the posterior end of the arc. (C) Implementation of the ridge. See Materials and methods for details.