A wonderful Red Turban Snail (Pomaulax gibberosus), photographed in the rocky intertidal zone in northern Sonoma County on 25 February 2017. This juvenile snail was only ~1 cm across. (Adults can reach diameters of over 5 cm.) Look for the distinctive nodules along the body whorl (photo above) and the spiral cords visible on the base of the shell (photo below).
Unlike many snails in this region, Red Turban Snails have a calcareous operculum ("trap-door"). And note that the operculum is very smooth:
P.S. For a view of a different species with a calcareous operculum, review the Pheasant Snail post from 20 December 2015.