Keystone from the fourth/fifth/sixth-century synagogue made out of limestone. Note the wreath that encloses a shell.
The double knot at the bottom of the wreath was held by two birds, which have been defaced, but traces of them can still be seen. For example, the one on the left is facing right, and (from the bottom left upward) one can make out its tail, breast, head, and beak.