Adblocker detetcted! It seems you are using an adblocker. Ads can be annoying, but small sites like this could not exist without some revenue from advertising. Your adblocker is also blocking the text content of this page. To view this page correctly, please turn off your adblocker.
Almost circular smooth shell, dull brown, grey or yellowish, usually with brown markings. The inside of the shell has a row of small 'teeth' along the hinge line. Up to 2½ inches.