This crab shell is very worn around the edges, and unfortunately the points between the eyes have disappeared. Now were there three rounded points as in the common shore crab Carcinus maenas, or possibly a whole row of smaller points, as in the velvet swimming crab, Necora puber?
Look closely at the photo, and there are hints of ground down multiple points. It suggests Necora puber.
But there is an absolute determining feature which rules out Carcinus maenas I even checked my own private collection of preserved specimens to confirm it.
Look at the rear of the crab shell, where the back legs would have been. It is not "rounded" as in Carcinus maenas, but is flat, with two definate "cut-off corners". These cut off corners are lengthened in Necora puber because it is a swimming crab. These widened corners of the rear of the carapace facilitate the wide degree of movement needed in active prolonged swimming.
This is a worn specimen of the carapace of the Velvet Swimming Crab, Necora puber.
In life, its carapace is covered in fine "velvet". Its agression is matched by its demonic red eyes. View the eyes in sunlight, in water, (instead of examined out of the water) for the full red-eye feature! Couple this with highlights of electric blue on chelae and leg joints, and you have a very beautiful crab.