- 42 A similar decline occurs in ratios of radioactivity in the tissues of the shore birds. However, the ratio of the mostactive tissue to the least active on any given date is usually about twice that found for the terns. In addition, the points in the decline of ratio with increasing time are more variable in the shore birds than those foundin the terns. The rate of decline of radioactivity in muscle, liver, and bone of north Rongelap terns is of two general types, logarithmic and semilogarithmic. A logarithmic decline as used here refers to a decline best described by a straight line on log-log paper. A semilogarithmic decline is best described by a straight line on semi-log paper (Fig. 13). type of curve (r = t72- 49) Bone exhibits the former Aigo thallecay curve for samples of bone taken on March 26 and July 16 is logarithmic (r = t71-85) In liver and muscle the decline is semilogarithmic with a half life of approximately 40 days. In the October collections the average radioactivity from 7 samples each of muscle and liver suggested that the decline in activity was beginning to assume a longer half life than 40 days (Fig. 13). However, a straight line (half life of 40 days) extended through the averages of the previous collections falls within the 95 percent confidence limits of the October values. The deviations of the latter points from the decline curve derived from the previous collections are therefore probably not significant. The radioactive decay of three liver samples from north Rongelap terns taken on March 26, 1954 is logarithmic (r = t7 2-83). Shore birds taken at Rongelap Island in October 1955 continued to contain higher levels of activity than terns. Ratios of shore bird radioactivity to those of northern Rongelap terns were as follows: muscle, 2. 9:1;