Deemed concealment for materially understated income creates presumption unless taxpayer proves absence of fraud or gross neglect. The amendment omits 'deliberately' from clause (c) of section 271(1) and inserts an Explanation deeming concealment where returned total income is less than eighty per cent of the total income as assessed under specified assessment sections (reduced by bona fide disallowed expenditure), unless the person proves the understatement did not arise from fraud or gross or wilful neglect.