PHENACOLEPADIDAE or CAPULIDAE or something else
Location: Gab Gab Beach, Guam
Size: 21mm
any idea what species?
Looks like Phenacolepas crenulatus (Broderip), as shown in Poppe's Philippine Marine Mollusks, and several other sources. However, there are a number of online sources that show a somewhat different shell as crenulatus.