480 L. D. BURLING CAMBRIAN AND ORDOVICIAN BRACHIOPODA 



The apparent synchroneity of (1) tlie iiiaugnratiou of major strati- 

 g-raphic units, (2) the introduction of new .and temporarily persistent 

 organic types, (3) the expansion of sea areas, and (4) the successive 

 deposition of finer sediments supports the tabular indication that the 

 majority of the species which appear to have been able to accommodate 

 themselves to changes in the character of the sea-bottom are the ones ac- 

 companying gradations from the more clastic to the less. The figures 

 for the more widely scattered occurrences (columns 3, 4, and 5) are 

 naturally less significant than those for individual sections (column 2), 

 but they appear to be corroborative, and thus bear evidence of a certain 

 dependability. Even with a due appreciation of the uncertainties in- 

 volved, we appear to be justified in assigning to the figures of this table 

 and to those of Table III a more than coincidental origin. 



