I couldn't get the picture to do it justice. Maybe a picture can't really do it justice.
That opening is something like 1.5 inches in diameter. It looked so odd that I didn't realize it was a barnacle at first, though I was able to get this image where it's obvious.
Wouldn't you think that bright orange margin would make it trivial to look up what type it is? Never seen one like that. ...it's on a holdfast, and we've had enough months of summer now that I couldn't remember which kind of kelp is which. But kelp season will soon be upon us again...
Those two "teeth", presumably the ends of those plates -- a lot of them seem to form their protective structures out of a lot of plates, anyway -- were a bit purplish.