Red-Brown Ark
Barbatia domingensis (Lamarck, 1819)
As identified by photos online, I have a dozen of these arks. There seems to be some discussion of the genus (see remarks below, also for an alternate name see Barbatia cancellaria) as found online. The shells I have are dark brown (1-2")with a light are that extends from the umbo to the bottom. For a good example see also Florida's Living Beaches p. 114.