(a) A tax of 2 ½% is imposed on inheritances which fall from husband to wife, from wife to husband, from ascendants to descendants or from children to parents, which, for purposes of this section, shall be described as Class I inheritances.
(b) For purposes of computing the tax on Class I inheritances, the taxable amount of such inheritances shall be determined by deducting from the total value thereof an exemption of $50,000, prorated to each person receiving a Class I inheritance in the ratio that each said inheritance bears to the total amount of all Class I inheritances.