Excise exemption for chewing tobacco and snuff limits duties to an ad valorem ceiling and prescribes apportionment. Chewing tobacco and snuff under the specified tariff sub headings are exempted from excise and additional excise duties to the extent those duties exceed an ad valorem ceiling of 25 per cent; the duty collectible up to that ceiling is to be apportioned between the Tariff Schedule duty and the additional excise duty in a 75:25 ratio.