Fueled by a strong stock-buying climate, total market value of the  nation's top 100 banking companies rocketed 13.2% in the second quarter, to   a record $331.4 billion on June 30.   
Data from the American Banker quarterly survey of banking industry  market capitalization appear on page 22. 
  
New York's Citicorp solidified its No. 1 ranking, gaining 35.8% to reach  $23 billion of market capitalization. The market value of U.S. Bancorp,   Portland, Ore., fell 7.5% to $2.4 billion, the sharpest decline in the   group.     
MBNA Corp., Newark, Del., ranked first in market value as a percentage  of total assets. Its $5 billion of capitalization equaled 48.5% of assets.   Bankers Trust New York Corp. was last in this category, with its $4.9   billion of market value equaling just 4.5% of assets.