Firstly they have a calcareous base, so they can't be the common East Coast species Semibalanus balanoides. The tubes in the shell have transverse septa, and also the shape of the orifice and the appearance of the operculum plates. I couldn't decide if they were A.subalbidus or A.eburneus, and went with the former. But now that you've got me looking at them again, I think that they probably are A.eburneus because of their more cylindrical shape. Those two species are very closely related.