What is the X in X-mas?
any member of the phylum Brachiopoda, a group of bottom-dwelling marine invertebrates. They are covered by two valves, or shells; one valve covers the dorsal, or top, side; the other covers the ventral, or bottom, side. The valves, of unequal size, are bilaterally symmetrical; i.e., the right and left sides are mirror images of one another. Brachiopods (from the Greek words meaning "arm" and "foot") are commonly known as lamp shells because they resemble early Roman oil lamps.