Sculpting a Mermaid Character with Dynamesh - ZBrush Creature Design: Creating Dynamic Concept Imagery for Film and Games - page 416

Figure 7-43: The fins dynameshed into the sculpture
6. At this stage, the fins are simply concentric rings leading up the neck. I want to add some interest to
them by changing the negative spaces they create. I will do this by subtracting some volume from the fins
and thereby giving some complexity to their form. Select the InsertSphere brush. Turn on Z symmetry and
draw two spheres while holding down Alt. Transpose scale them to intersect with the neck fins as shown