DAVENPORT. — EVOLUTION OF PECTEN. 



131 



ventral and antero-posterior axes. We must now combine these two 

 systems by determining the relation of the dorso-ventral diameter and 

 the transverse. This relation is set forth in Table V. 



The ratios given in Table V above register the (half) globosity of the 

 mollusc. The difference in globosity between the Eddystone and the 

 Irish Sea forms is not marked, but the Forth shells are decidedly flat, 

 There is no clear change in globosity with age. 



All axes of the molluscan body are of course correlated. We have 

 seen that this correlation becomes extraordinarily high in axes that play a 

 physiologically similar part. What is the correlation between the dorso- 

 ventral and transverse diameter of the right valve, which play very dis- 

 similar parts ? 



TABLE VL 



Correlations between Dorso-ventral Diameter and Partial Transverse 

 Diameter of Right Valve (Appendices D, E, F) 



The difference between the correlation coefficient of Eddystone and 

 that of the Firth of Forth shells is probably not significant; but the 

 Irish Sea shells have sensibly the least correlation between the axes in 

 question. 



