If you look at the top of this, you might see that the early juvenile shape would be nearly flat-topped and more discoidal, even resembling a vitrinellid or architectonicid. I believe I encountered some of the juveniles before finding this one, so it's gratifying to see an adult "outcome". 3mm diameter.

Leptothyra and Homalopoma are two interchangeable names, I've been told.