i68 



THE AMERICAN MIDLAND NATURALIST 



tAble II. 

 vSEXUAL DIMORPHISM IN NAYADES. 



(Dextro-sinistral diameter 



DvSD 



or convexity of valve divided 



(Dorso-ventral diameter of — j-— value divided by length 

 giving relative height.) 



1 



(Distance posterior to extremity -j^ from aline passing through median dor- 

 sal plane of valve expressing relative \degree posterior development, shell.) 



(Distance anterior to extremity -j^ from a line passing through median dor- 

 sal plane of valve expressing relative degree anterior development shell.) 



