No. 634] BIONOMICS OF MELL1TA 



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changes somewhat, however, from time to time, being more 

 muddy and less sandy in some years than in others. One may 

 nevertheless entertain the idea that genetic factors are con- 

 cerned in determining the growth-changes in the body-shape of 

 some individuals. This matter should be studied in a larger 

 series of specimens than I have been able to secure in the time 

 devoted to this work. Especially interesting is the fact that this 



species occurs in the fossilized state above the beach zone along 

 the shores of islands in Great Sound. The seven large fossil 

 tests I have examined were all of the almost perfectly circular 

 type (cf. Fig. 1). 



In order to follow more precisely the course of growth-changes 

 in Mellita, I endeavored to derive the curve of its growth, and to 

 fix the average duration of its life. The results, which are pro- 

 visional only, are given in Fig. 5. Specimens were measured 

 from one locality— Cobbler 's Cut, Spanish Point— in September. 

 The transverse diameter was measured, since it is less subject 

 than is the antero-posterior to fluctuations induced (a) by the 

 tendency, already noted, to form a projection at the anterior 

 antimere, and (b) through injuries suffered at the posterior 

 inter-radius. From the modes in the frequency distribution of 

 sizes, it was deduced that at 6-months M. sexies-pcrforata meas- 

 ures 2.2 cm. in transverse diameter; at 1.5 years, 5.7 cm.; and so 

 on, as indicated in Fig. 5. It seems possible that the average 

 duration of life is about 4 years, according with the fact that 



