THE OSTEACODA. 



225 



Stage II. 

 Mean, 2'34 mm. 



2- 

 f 3 



I 

 I 1 



I 1 



Stage III. J 

 Meau, 1-50 mm. I 



17 



Stage IV. 



11 



Mean, 0*96 mm. j q 



Stage V. 

 Mean, 0'66 mm. 



mm. 

 2-5 

 2-4 

 2-3 

 2-2 

 2-1 

 20 

 1-9 

 1-8 

 1-7 

 •6 

 1-5 

 1-4 

 1-3 

 1-2 

 1-1 

 10 

 0-9 

 0-8 

 0-7 

 0-6 



J. 



] Stage II. 



ean, 201 mm. 



11 



1 I Stage III. 



7 I Mean, 148 mm. 



5; 



14 



Stage IV. 

 Mean, 1"0 mm. 



Table A.— Total specimens measured: 65 ? + 36 c? =101. 



It will be noted that the total numbers are very small in both sexes, but higher totals 

 are available in some other species. 



Now, taking the mean lengths of the females at the different stages : — 0'62xl'56 = 

 0-96; 0-96 Xl-56=l-497 (mean 1-50); I'SOx 1-56= 2-34,— that is to say, the mean 

 length of the shells at each stage when multiplied by (what we may call) the growth- 

 factor 1'56 * yields approximately the mean of the next oldest stage. The males are 

 much fewer and therefore respond less accurately: — 10xl'45 = rjj; l"4x l'45 = 2-03. 

 In other words, at each of the growth-moults, the shells of the females increase by 

 some 56 per centum of their length, and those of the males by some 45 per centum. 

 The decimals would no doubt require alteratioa (especially in the males) if more 

 specimens were available ; as already said, the case of imhricata is less conclusive 

 mathematically than zoologically. Even in species more plentifully available it is 

 hardly possible to get the second place of decimals in the observed mean to coincide 

 precisely with that in the mean calculated from the growth-factor ; the reason being 

 that it is impossible to measure these little biconvex and delicate shells with any 

 pretence at accuracy to a hundredth of a millimetre, which is tiie second place of 

 decimals in this instance. 



The observed lengths and their frequency have been plotted on scale-paper in the 

 accompanying diagram (fig. A, p. 22G). The abscissa represents a millimetre-scale from 

 0"6 to 2'7 mm. ; the ordinates on the left of this represent the observed numbers of 

 females, the ordinates on its right those of the males. Even witli the small total 



* Determined empirically on a slido-nile. 



