434 L. D. BURLING CAMBRIAN AND ORDOVICIAN BRACHIOPODA 



frequent both facies.of an interbedded series; (h) that of the 45 species 

 just mentioned the number occupying positions in the stratigraphic se- 

 quence where the gradation upward is from the more clastic to the less 

 is from two to five times that of species showing the reverse progression ; 

 and {{) that after dividing the 1,460 localities into three entirely distinct 

 and unrelated groups there are obtained for each of the groups and for 

 eacli of the types of sediment average figures which bear striking mathe- 

 matical evidence of the reliability of their indication that while the num- 

 ber of species per locality is remarkably uniform it is smaller in shale 

 tlian in sandstone and greatest in limestone. 



