clamshell–the mouth or lips. Example: “Shut your clamshell and let me talk!” People also would shorten it to clam (1825), but clamshell seems to have been more widely used. From 1832 on. According to Bartlett’s Americanisms it was most common in Connecticut and Rhode Island.
Also, happy as a clam–extremely joyful, sitting pretty. My mother used to say this a lot, so I assumed it was pretty much a 20th century thing. Apparently I am wrong. It goes back to 1834.