The upper surface of the terrapin's shell, called the carapace, is formed of 13 plates, or scutes. Each scute is roughly circular and grooved with rings. Since one ring is formed each year, the age of the turtle can be determined by counting the rings of growth, just as in a tree. The female turtles get to be nearly twice the size of the males. A large female may be 9 inches in length, whereas a large male rarely exceeds 5 inches. Diamondbacks were severely over-collected in the last century for turtle soup. One hundred years ago, they sold for a dollar an inch, and the larger, breeding females were more valuable. The collection of breeding females quickly reduced the population. Although some local populations have recovered, they are still not as common as they once were. This slow recovery is due, in part, to the turtles' own biology. Maximum egg production does not occur until the females are 25 years old.