FIGURE 25. A classical view of mantle shape among the four taxa. Each taxon is represented only by its linear regression line of valve breadth versus length (thick colored lines); these are the same lines as shown in Fig. 20. This morphospace has been contoured (light blue lines emanating from the origin) by two sets of standard shape descriptors based on length:breadth ratios: those of the Systematics Association (Stearn 1973) and Cox (1991); these ratios are shown at the ends of the contours furthest from the origin. The intersection of the regression lines and the black dashed vertical line indicates the length-to-breadth ratio of each taxon at valve length = 100 µm. The light brown dashed line passing through the origin has a slope of 3.3:1 and thus represents a constant shape from the classical perspective. The 95% prediction intervals are the same as for valve breadth (Fig. 20).