Growth almost stops after the thirteenth year in females and after the
eleventh or twelfth year in males, approximately three years, on the
average, after sexual maturity is attained. The oldest individuals in
which plastral length had increased measurably in the season of
capture were females 14 (2 specimens) and 15 (1) years old. The age of
the oldest growing male was 13 years.
The germinal layer of the epidermis probably remains semiactive
throughout life but functions chiefly as a repair mechanism in adults
that are no longer growing. Growth-rings continue to form irregularly
in some older adults. Growth-rings formed after the period of regular
growth are so closely approximated that they are unmeasurable and
frequently indistinguishable to the unaided eye. If the continued
formation of growth-rings is not accompanied by wear at the edges of
the laminae, the laminae meeting at an interlaminal seam descend, like
steps, into the seam (Pl. 22, Fig. 2). Interlaminal seams of the
plastron deepen with advancing age in most individuals.
Some individuals that are well past the age of regular growth show
measurable increments in years when conditions are especially
favorable. The three oldest growing females were collected in 1954--an
exceptionally good year for growth. Allowing some latitude for
irregular periods of growth in favorable years subsequent to the
period of regular, more or less steady growth, 15 to 20 years is a
tenable estimate of the total growing period.