I was just wondering if it could be another family group... Myidae (i.e Mya arenaria) presents similar elliptical shape and the umbo is located close to the shell center.
Do you have a close up picture of the internal? The ligament looks tellinoidean and there are some Psammobiidae that are estuarine. Soletellina perhaps?
There are 2 cardinal teeth, no lateral teeth and a prominent ligament on a flared nymph (the shelf-like protrusion beneath the ligament) and the pallial sinus is deep and is confluent (runs together) with the pallial line for the ventral part. This is a Psammobiid for sure but because the valves are worn I cannot tell which genus for sure. Soletellina is the closest that I can see.