Some people consider that the presence of this species in estuaries such as the Hudson River (its most northerly occurrence), represent relict populations from the Pleistocene period.
Other people consider it more likely that this species is an introduction, and that the native range is thought to be the Gulf of Mexico, although back in the warmer part of the Pleistocene era, it did live all up the East Coast of the US.
This species can survive only in estuary areas, because it requires low salinities.
I found it once before a few days previously on the same little beach on Ward's Island, so it was pretty exciting for me to find it again today!
Here are the previous valves which I found 4 days before:
