Ontogenetic changes in the shell can be summarized as follows: 1) Buttresses become less distinct in the first two years of life (plastral lengths of 40 to 55 mm.); 2) Interdigitating processes of the forelobes and hind lobes become relatively shorter and wider, the entoplastron no longer projects posterior to the hinge, the lateral apex of the pectoral lamina becomes creased, and some movement of the plastron can take place between the second and third years (plastral lengths of 55 to 65 mm.); 3) Plastral lobes become freely movable upon one another and upon the carapace by the end of the fourth year (plastral length approximately 70 mm.) in most individuals. The plastron of a juvenal box turtle is not completely immovable. The bones of the shell are flexible for a time after hatching and allow some movement of the plastron; but the relatively greater bulk of the body in young box turtles would prevent complete closure of the shell even if a functional hinge were present. Hatchlings can withdraw the head and forelegs only to a line running between the anterior edges of the shell. To do so the rear half of the shell is opened and the hind legs are extended. When the head and forelegs are retracted to the maximum, the elbow-joints are pressed against the tympanic region or behind the head; the fore-limbs cannot be drawn part way across the snout, as in adults. Hatchlings can elevate the plastron to an angle of approximately nine degrees; the plastron of an adult, with shell closed, is elevated about 50 degrees. Hatchlings flex the plastron chiefly in the region of the humeropectoral seam, rather than at the anlage of the transverse hinge. Adult box turtles, when walking, characteristically carry the forelobe of the plastron slightly flexed. This flexion of the plastron, combined with its naturally up-turned anterior edge, cause it to function in the manner of a sled runner when the turtle is moving forward. A movable plastron, therefore, in addition to its primarily protective function, seems to aid the turtle in traveling through tall grass or over uneven ground. The gular scutes, on the anterior edge of the forelobe, become worn long before other plastral laminae do. An adult female from Richland County, Illinois, had an abnormal but functional hinge on the humeropectoral seam in addition to a normal hinge on the pectoroabdominal seam. The abnormal hinge resulted from a transverse break in which ligamentous tissue later developed. The muscles closing the plastron moved the more anterior of the two hinges; the normal hinge was not functional. Color and Markings The markings of the shell change first when postnatal growth begins and again when sexual maturity is attained. They are modified gradually thereafter as the shell becomes worn.