Models III 17 and 18 are (elliptic) cones. They only show half of the cones - the cone really ought to have an identical second piece lying above it. Model III 18 displays lines of curvature. Recall that any umbilic points are "absorbed into the singularity" in the centre, so to speak.

Images: Model III 17; Model III 18 (from above); Model III 18

Model III 17Model III 18 - Lines of curvature (from above)Model III 18 - Lines of curvature

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