Hi, Andrew. I checked out the other shells you had mentioned besides
Tellina Magna. I have foond both Papyridea soleniformis, and semele
purpurascens, (fine pairs ) diving off both FL coasts.  If I
identify it correctly, I do think I've found psammotreta intastriata
just wading on various mud and sand flats on the west coast ( but
I could be wrongin my ID).  Anyway, it does look as if diving helps.
 
Karen VV