What's the Difference Between a D and A?

The shape of the pottery and what we know about the nage that's on board, makes julius caesar a legitimate candidate for who commissioned the boat. But do they have a flat bottom, or a conical bottom? They have a conicalbottom. Why? Cause i've seen it drawn both ways. And so maybe they just built things to accommodate that shape. Because they didn't have to stand up. Seems like a flat bottom would make a lot more sense. I could see how the cone bottom could be used, based on other things. I've seen a just like it. It might be wrong, butl shus go for it.