to my knowledge this except the valves of the brachiopods remain contiguous after death. these specimens look like lamellibranchs despite their valves are joined
Your documented specimen shows close affinity to genus Alectryonia or Lopha, a shallow marine bivalve thriving in high energy coaster waters. The architecture is well designed to withstand wave and current energy. This is stationary epifaunal suspension feeder and the rugged architecture is superbly designed, as the smaller valve remains cemented to some hard substrate.
In Brachiopods both the valves are firmly cemented together. Best