No applying bank shall be admitted to membership unless it possesses capital stock and surplus which, in the judgment of the Board of Governors of the Federal Reserve System, are adequate in relation to the character and condition of its assets and to its existing and prospective deposit liabilities and other corporate responsibilities: Provided, That no bank engaged in the business of receiving deposits other than trust funds, which does not possess capital stock and surplus in an amount equal to that which would be required for the establishment of a national banking association in the place in which it is located, shall be admitted to membership unless it is, or has been, approved for deposit insurance under the Federal Deposit Insurance Act [12 U.S.C. 1811 et seq.]. The capital stock of a State member bank shall not be reduced except with the prior consent of the Board.