A couple of months ago, I was counting crabs' teeth, reviewing the crabs we've collected on our local beaches, in the intertidal level.
There are always more, of course. Some live in the lower intertidal zone; many are subtidal. We won't find those, unless the molts get washed up. Or unless we take up scuba diving. (I wish we could!)
Close to the lower edge of Boundary Bay intertidal zone, we found this carapace:
As far as I know, this is the first helmet crab I've seen, probably because they live in eelgrass and kelp beds, hiding in the shadows from galumphing human waders. And they're "one of the fastest moving Pacific Northwest crabs". (MLotPN)
This little crab, under 5 inches long at maturity, has 6 teeth along each side, starting the count at the back of the eye. With the crab alive and running about, they'd be a bit harder to count than they are here, because he's also really hairy. NOAA has a photo of one; try your hand. Can you see all 6? I can't.
Ours is a youngster. Besides being only about an inch long, he still has a touch of green on the carapace. A mature crab could be red or brown or orange; kelp colours, for maximum invisibility.