Abdominal laminae were measured at the interlaminal seam; since the laminae frequently did not meet perfectly along the midline (and were of unequal length), the right abdominal was measured in all specimens. Growth-rings on the abdominal laminae were measured in the manner shown in Plate 22.
Data were obtained for an aggregate of 1272 seasons of growth in 154 specimens (67 females, 48 males, and 39 of undetermined sex, chiefly juveniles). Averages of calculated plastral length were computed in each year of growth for specimens of known sex (Figs. 9 and 10) and again for all specimens examined. Annual increment in plastral length was expressed as a percentage of plastral length at the end of the previous growing season (Fig. 11). Increment in plastral length for the first season of growth was expressed as a percentage of original plastral length because of variability of growth in the season of hatching; growth increments in the season following hatching are, therefore, not so great as indicated in Figure 11.
Areas of new laminal growth were discernible on laboratory hatchlings soon after they ate regularly. Hatchlings that refused to eat or that were experimentally starved did not grow. The first zone of epidermis was separated from the areola by an indistinct growth-ring (resembling a minor growth-ring) in most hatchlings, but in a few specimens the new epidermis appeared to be a continuation of the areola. Major growth-rings never formed before the onset of the first hibernation.
Growth in the season of hatching seems to depend on early hatching and early emergence from the nest. Under favorable conditions hatchlings would be able to feed and grow eight weeks or more before hibernation. Hatchlings that emerge in late autumn or that remain in the nest until spring are probably unable to find enough food to sustain growth.
Sixty-four (42 per cent) of the 154 specimens examined showed measurable growth in the season of hatching. The amount of increment was determined in 36 specimens having a first growth-ring and an areola that could be measured accurately. The average increment of plastral length was 17.5 per cent (extremes, 1.8-66.0 per cent) of the original plastral length. Ten individuals showed an increment of more than 20 per cent; the majority of these individuals (8) were hatched in the years 1947-50, inclusive.