Fossil Bracliiopocl Platystrophia lynx. 



13 



polygons of seventeen, eighteen, nineteen, and twenty plica- 

 tions on the ventral valve. Therefore any shell with four 

 plications in the sinus should have between seventeen and 

 twenty (both numbers inclusive) plications on the valve. It 

 should have no more and no fewer. 



An inspection of our material does not show this to be the 



IV V 



a b 



IV 



Ilia 



III 



Fig 1. Curves of Groups I to V. Ou tlie axis of ordinates, each small 

 division represents four individuals. On the axis of abscissas, each fifth (as 

 from a to b) represents one class. 



case. In a series of 153 specimens with four plications in the 

 sinus, the range is from 16 to 26 plications on the ventral 

 valve. The average is 18-8 plications. The specimens with 

 two plications in the sinus have from 11 to 22 plications on the 

 valve. Those with one plication in the sinus have from 10 to 

 15 on the valve. The correlation is approximate rather than 

 precise. 



II 



