This is a chiton, a member of the mollusc family, not, as I initially thought, a portal for aliens who will take us away on 12.21.12. We were on the Mayan Riviera after all.
Also, the Mexicans call them Cucaracha de Mar.
Here is some more useful information:
|Chitons have an eight-piece shell. The hard pieces are called valves. The leathery flesh is called the girdle and the girdle holds the valves together. There are about eight hundred kinds of chitons in the world. They cling to rocks. Their foot is a wide muscular organ that helps them hold on to rocks. They are very sluggish. They eat algae. They eat with their radula. Radula is a ribbon of hard teeth. The chitons grow 1/3 to 12 inches.|