The patellids living higher in the tidal zone try to lose superfluous heat by increasing their surface. For that they use the many ribs and ridges on the shell. Patellids with a smooth surface give away an environment amidst waves and streaming water. In this, often turbulent, environment high conical shells are not advisable. A more streamlined shape causes less turbulence.
Individuals belonging to one species can differ considerably with regards to their environment.
Photographs Ron Voskuil, NL-Delft.