3. Results
3.1. Shape and internal structure
The mineralized tooth of both limpets and chitons consists of the mineralized cusp proper and a rod-shaped base composed of organic material. The base lies parallel to the radular ribbon and is attached to this ribbon; the cusp is perpendicular to the base. The ratio of maximal length over maximal width of the cusp in the chitons studied is about 1. In the limpet studied the same ratio is measured in the three-pronged cusps, while in the one-pronged cusp this ratio is about 3. The cusp is enclosed by two surfaces: the concavely bent leading surface, i.e., the surface that leads during the grazing stroke, and the convexly bent trailing surface. To show the variation in shape of the cusp in a number of species, we used a stereo-micrometer to accurately record the shape of both the leading and trailing surfaces in a sagittal plane intersecting the very tip of the cusp and the tooth base. Some of the results are depicted in Fig. 1. Also, the outlines of the teeth depicted in Fig. 9 are based on the results of the stereo-micrometer measurements. As a measure for the shape we defined `curvature' of the cusp surface as the angle between the tangent near the tip of the cusp and the tangent near the base in the stereo-micrometer recordings. It appeared that in chitons the curvature of (1) the leading surface varied from 70° to about 90° and (2) the trailing surface from 40° to about 90°. In the limpet studied both the leading and the trailing surface showed a curvature of more than 100°. In all species strongest curvature was apparent in the lower half of the leading surface. In chitons the leading surface may slightly be recurved close to both the base and the tip of the tooth. The profiles of both the leading and the trailing surface in juvenile and adult specimens of the same species are identical (Fig. 1, shown for the leading surface only). This shows that the curvatures of these surfaces are species-specific traits. Dependent on the species the boundary plane between cusp and base is either subparallel to the elongation of the base (Fig. 9A) or makes a large angle with the elongation of the base (Fig. 9b and C).