Mantle is in general a bad way to ID. But if you have been around clams for a while you can make a pretty accurate ID based on the mantle. I would say this is a maxima from the mantle shape. The large, rounded "lips" on the mantle are very maxima-esque. Maxima shells have more ridges when they are smaller, when they are bigger the ridges are pronounced near the top of the shell, near the bottom it is more smooth. From this top down shot you are looking at the bottom of the shell and I'm willing to bet there are ridges on the shell hidden from view by the mantle. However, a side shot showing the shell would make for a more sure-fire ID.