Ampyx is a Trilobite. The picture here is probably about twice natural size. Trilobites first appeared in the Upper Cambrian period, about 500 million years ago. They proliferated in the shallow marine realm, grubbing about near the bottom of shallow shelf seas. They were early arthropods, and were ancestors of modern shrimps, crabs, spiders, scorpions, centipedes and insects. They had legs, loads of them, but they are very rarely preserved. During a long and happy evolution they invented body-armour and eyes, and assumed bizarre shapes and habits including burrowing and swimming upside-down near the ocean surface. They were almost wiped out during the Devonian period (412-354 million years ago), but a few managed to hang on into the Permian (ca 250 million years ago). Because of their appearance during the Cambrian period (named, you may guess, after Wales) they are emblematic of Welsh geology.
Fossil trilobites are not uncommon in the right places, but are seldom the complete specimens that grace museum displays. This arises from their body-armour (cuticle) which was fine for keeping predators out, but became a trifle restrictive if you wanted to grow. Then it was necessary to shed one's old cuticle and put on a rapid spurt of growth before the new one hardened (just like modern crabs). It was also advisable to keep a low profile while this was going on, in case of becoming a ready meal. Thus, many bits of cuticle were potentially available for preservation before the whole beast died and was buried (with luck) in the soft, oxygen-poor ooze on the sea floor. As layers of ooze accumulated, the lower layers eventually became hardened into rock, and ultimately the collision of continents pushed the deep-down layers back up to the surface in crazily folded layers, still enclosing the imprint of their long-dead creatures.
The toughest bits of the body armour were the head shields, which is handy because these are the most distinctive parts. Ampyx takes its name from the ancient Greek word for a style of broad headband that covered much of the forehead. Here is a posh, Greek lady wearing her ampyx.........She is the nymph Terina who gave her name to a colony in Italy. She was the protector of the local spring, and appeared on the back of the city's coins, always with an elaborate hair-do. (Thanks to Lawrence University, Appleton Wisconsin, USA for that).
Trilobites are incredibly useful for comparing rocks of different ages and geological provinces, because they evolved rapidly into recognisable local populations. Also, the cuticle was tough enough to survive in more energetic environments, which destroyed Graptolites (someone else will have to explain them), which are used in the same way to compare and correlate rocks.